wl1251: Add vif checking to rssi and beacon loss events

wl1251: Add vif checking to rssi and beacon loss events

The chipset is initialized and the vif for the interface are created
separately - there is a small window, when the interface is configured,
that the firmware sends rssi or beacon loss events while the vif is not yet
existing, causing an oops in the mac80211.

Add checking for the vif to the handlers, ignoring the events if there is not
yet a vif.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Yuri Ershov <ext-yuri.ershov@nokia.com>