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 <bob@cozybit.com>
wcn36xx: initialize skb_lock

ctl->skb_lock is never initialized, a fact caught by lockdep.

Signed-off-by: Bob Copeland <bob@cozybit.com>
wcn36xx: initialize dxe lock

The dxe lock is missing its initialization, so add it.

Signed-off-by: Bob Copeland <bob@cozybit.com>