wlcore: consolidate free_link and always call it

Make sure free_link is always called when removing an interface. This
ensures all skbs belonging to this interface are returned to mac80211.
Otherwise these dangling skbs might crash the system on the next
call to wl1271_tx_reset_link_queues(). This happens on recovery/stop or
an unsuccessful Tx flush.

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