wcn36xx: Check DXE IRQ reason

IRQ reason was not cheked for errors.
Although error handing is not currently supported, it
will be nice to output an error value to the log if the
DMA operation failed.

Signed-off-by: Ramon Fried <rfried@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>