wcn36xx: split DMA mask register writing.

Per comments from Bjorn Andersson <bjorn.andersson@sonymobile.com>,
split DMA mask register writing as seperate patch in case we need
bi-sect in the furture.

Signed-off-by: Fengwei Yin <fengwei.yin@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>