wifi: ath12k: fix DMA unmap warning on NULL DMA address

wifi: ath12k: fix DMA unmap warning on NULL DMA address

In ath12k_dp_tx(), if we reach fail_dma_unmap due to some errors,
current code does DMA unmap unconditionally on skb_cb->paddr_ext_desc.
However, skb_cb->paddr_ext_desc may be NULL and thus we get below
warning:

kernel: [ 8887.076212] WARNING: CPU: 3 PID: 0 at drivers/iommu/dma-iommu.c:1077 iommu_dma_unmap_page+0x79/0x90

Fix it by checking skb_cb->paddr_ext_desc before unmap it.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0-03427-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.15378.4

Fixes: d889913205cf ("wifi: ath12k: driver for Qualcomm Wi-Fi 7 devices")
Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230830021131.5610-1-quic_bqiang@quicinc.com