wl1251: only call ieee80211_beacon_loss in managed mode

ieee80211_beacon_loss() is only to be called in managed mode,
but the firmware may send the sync timeout event at any time,
so do a check before calling.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>