wifi: ath12k: add the missing RCU lock in ath12k_dp_tx_free_txbuf()

wifi: ath12k: add the missing RCU lock in ath12k_dp_tx_free_txbuf()

RCU read lock is missing in ath12k_dp_tx_free_txbuf() before calling
ath12k_dp_to_pdev_dp(), causing below warning:

WARNING: suspicious RCU usage
-----------------------------
drivers/net/wireless/ath/ath12k/dp.h:653 ath12k dp to dp pdev called without rcu lock!
Call Trace:
 <IRQ>
 show_stack
 dump_stack_lvl
 dump_stack
 lockdep_rcu_suspicious.cold
 ath12k_dp_tx_free_txbuf
 ath12k_wifi7_dp_tx_completion_handler
 ath12k_wifi7_dp_service_srng
 ath12k_pci_ext_grp_napi_poll
 [...]
 </IRQ>

Invoke guard(rcu) to fix it.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.1.c5-00302-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.115823.3

Signed-off-by: Baochen Qiang <baochen.qiang@oss.qualcomm.com>
Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20251119-ath12k-fix-missing-rcu-lock-v1-1-8155de1dc4fc@oss.qualcomm.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>