wcn36xx: set DMA mask explicitly

The device takes 32-bit addresses only, so inform the DMA API about it.
This is the default on msm8016, so that doesn't change anything, but
it's best practice to be explicit.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>