wl12xx: AP mode - add AP specific event

Add STA-remove completion event. Unmask it during boot if operating in
AP-mode.
Ignore unrelated events in AP-mode.

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