wifi: ath12k: remove memset with byte count of 278528

Sparse warns:

drivers/net/wireless/ath/ath12k/dp.c:1471:15: warning: memset with byte count of 278528

There's no need to use memset() here, instead call dma_alloc_coherent() with __GFP_ZERO.

While at it, remove an extra line before the error handler.

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230222164014.860-1-kvalo@kernel.org