wifi: ath11k: avoid burning CPU in ath11k_debugfs_fw_stats_request()

wifi: ath11k: avoid burning CPU in ath11k_debugfs_fw_stats_request()

We get report [1] that CPU is running a hot loop in
ath11k_debugfs_fw_stats_request():

94.60%     0.00%  i3status         [kernel.kallsyms]                 [k] do_syscall_64
        |
         --94.60%--do_syscall_64
                   |
                    --94.55%--__sys_sendmsg
                              ___sys_sendmsg
                              ____sys_sendmsg
                              netlink_sendmsg
                              netlink_unicast
                              genl_rcv
                              netlink_rcv_skb
                              genl_rcv_msg
                              |
                               --94.55%--genl_family_rcv_msg_dumpit
                                         __netlink_dump_start
                                         netlink_dump
                                         genl_dumpit
                                         nl80211_dump_station
                                         |
                                          --94.55%--ieee80211_dump_station
                                                    sta_set_sinfo
                                                    |
                                                     --94.55%--ath11k_mac_op_sta_statistics
                                                               ath11k_debugfs_get_fw_stats
                                                               |
                                                                --94.55%--ath11k_debugfs_fw_stats_request
                                                                          |
                                                                          |--41.73%--_raw_spin_lock_bh
                                                                          |
                                                                          |--22.74%--__local_bh_enable_ip
                                                                          |
                                                                          |--9.22%--_raw_spin_unlock_bh
                                                                          |
                                                                           --6.66%--srso_alias_safe_ret

This is because, if for whatever reason ar->fw_stats_done is not set by
ath11k_update_stats_event(), ath11k_debugfs_fw_stats_request() won't yield
CPU before an up to 3s timeout.

Change to completion mechanism to avoid CPU burning.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.37

Fixes: d5c65159f289 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Reported-by: Yury Vostrikov <mon@unformed.ru>
Closes: https://lore.kernel.org/all/7324ac7a-8b7a-42a5-aa19-de52138ff638@app.fastmail.com/ # [1]
Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20250220082448.31039-2-quic_bqiang@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>