wl12xx: track freed packets in FW by AC

Track the number of freed packets in each AC when receiving an interrupt
from the FW. This paves the way for tracking allocated packets per AC.

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