staging: wfx: change the way to choose frame to send

The current code computes itself the QoS policy to choose which frame
should be sent. However, firmware already do that job. Firmware would
prefer to have packets in every queues and be able to choose itself
which queue to use.

So, this patch sort the queues from the emptiest to the fulliest (thanks
to the pending frames counter introduced a few commits earlier). It send
frame to the least full queue.

However, we continue to be careful with frames that have to be sent
after a dtim ("cab": Content After (DTIM) Beacon).

So, this patch splits AC queues in two skb_queues: one for normal frames
and another for cab frames. It cares to send frames from CAB skb_queue
if appropriate.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-23-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>