ath10k: use dma_zalloc_coherent()

use dma_zalloc_coherent() instead of dma_alloc_coherent and memset().

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>