wifi: ath12k: refactor transmitted arvif retrieval

wifi: ath12k: refactor transmitted arvif retrieval

Create a new function ath12k_mac_get_tx_arvif() to retrieve 'arvif'
for the transmitted interface of the MBSSID set. This clean up will
help modifying the same code path for MLO changes.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.3.1-00173-QCAHKSWPL_SILICONZ-1

Signed-off-by: Aloka Dixit <aloka.dixit@oss.qualcomm.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20250210182718.408891-4-aloka.dixit@oss.qualcomm.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>