wifi: ath12k: avoid multiple skb_cb fetch in ath12k_mac_mgmt_tx_wmi()

wifi: ath12k: avoid multiple skb_cb fetch in ath12k_mac_mgmt_tx_wmi()

ath12k_mac_mgmt_tx_wmi() fetches ath12k's skb_cb space multiple times from
TX skb which is redundant operation. Save the skb_cb in a local pointer
and use the same instead.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.3.1-00173-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: Rameshkumar Sundaram <rameshkumar.sundaram@oss.qualcomm.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20250415195812.2633923-3-rameshkumar.sundaram@oss.qualcomm.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>