mac80211: fix beacon loss detection after scan

Currently beacon loss detection triggers after a scan. A probe request
is sent and a message like this is printed to the log:

wlan0: beacon loss from AP 00:12:17:e7:98:de - sending probe request

But in fact there is no beacon loss, the beacons are just not received
because of the ongoing scan. Fix it by updating last_beacon after
the scan has finished.

Reported-by: Jaswinder Singh Rajput <jaswinder@kernel.org>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>