wcn36xx: don't process 'valid' descriptors

The DMA engine will reset the valid bit after a descriptor is
complete; any with the valid bit still set may still be in
use by the hardware, so check that before freeing the descriptor.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>