wifi: ath12k: refactor ath12k_reg_build_regd()

wifi: ath12k: refactor ath12k_reg_build_regd()

Currently we pass intersect flag to tell ath12k_reg_build_regd() whether
regulatory rulse are intersected. This flag is determined in
ath12k_reg_handle_chan_list() and has no other users.

Move related logic into ath12k_reg_build_regd() to make code clear. Also
relocate ath12k_reg_is_world_alpha() to avoid forward declaration, and
refine it for code simplicity.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20250418-ath12k-6g-lp-vlp-v1-3-c869c86cad60@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>