wifi: ath12k: Add num_stations counter for each interface

wifi: ath12k: Add num_stations counter for each interface

Currently, ath12k driver maintains a counter to store the number of
stations connected to each radio. However, at certain times like
debugging, it is useful to know number of stations connected to any
one of the interface in that radio.

Add support to maintain a counter for number of stations connected to
each interface.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.4.1-00199-QCAHKSWPL_SILICONZ-1
Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Aditya Kumar Singh <aditya.kumar.singh@oss.qualcomm.com>
Signed-off-by: Roopni Devanathan <quic_rdevanat@quicinc.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20250630044531.3490058-1-quic_rdevanat@quicinc.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>