wlcore: track FW-allocated packets per link

Move FW-allocation tracking code to the fw_status function and track
allocations made by all links. These will be incorporated in the
improved Tx scheduling algorithm.

Manually zero the system link counters on op_stop, as this link is not
allocated the normal way.

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