wifi: ath12k: Use skb->len for dma_unmap_single() length parameter

During dma_unmap_single() for extended skb, the driver currently uses
sizeof() the corresponding structure. When the allocation size changes,
one of the parameters of dma_unmap_single() needs to be updated everywhere.
Improve code readability by using skb->len instead of sizeof() the
structure.

Signed-off-by: P Praneesh <quic_ppranees@quicinc.com>
Signed-off-by: Raj Kumar Bhagat <quic_rajkbhag@quicinc.com>
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>