wl12xx: fix a memory leak of probereq template upon recovery

wlvif->probereq is zeroed when adding an interface but
the skb pointed to isn't freed when the interface is removed.
This would lead to a mem leak on every recovery.
Fix it by freeing the skb when removing the interface.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>