wifi: ath11k: wrap ath11k_mac_op_get_txpower() with lock-aware internal helper

wifi: ath11k: wrap ath11k_mac_op_get_txpower() with lock-aware internal helper

Refactor ath11k_mac_op_get_txpower() by introducing a new internal function
ath11k_mac_handle_get_txpower(), which assumes the caller holds the
appropriate lock. This prepares the codebase for future change where the
internal function may be invoked directly with the lock already acquired,
improving modularity and lock handling consistency.

No functional change intended.

Tested-on: QCN9074 hw1.0 PCI WLAN.HK.2.9.0.1-02146-QCAHKSWPL_SILICONZ-1

Signed-off-by: Aditya Kumar Singh <aditya.kumar.singh@oss.qualcomm.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Reviewed-by: Baochen Qiang <baochen.qiang@oss.qualcomm.com>
Link: https://patch.msgid.link/20251017-add_tx_power_insertion_support-v1-2-f08feacfca93@oss.qualcomm.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>