wl12xx: implement Tx watermarks per AC

Each AC is stopped when its queue is filled up to the high watermark,
and restarted when its queue it lower than the low watermark. This
ensures congested ACs are not able to starve other ACs.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>