wl1271: Check vif for NULL when indicating beacon-loss

Because the interface is started and the vif are created and destroyed
separately, there is a slim possibility beacon-loss indications occur while
there is no vif - causing a kernel-oops unless checked.

Add checking for the vif.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>