wlcore: enable beacon filtering only after receiving a beacon

Enabling beacon filtering before receving a beacon
might result in not having a beacon at all for the
current connected AP, which prevents the station
from entering power-save.

Replace the current approach (of starting beacon
filtering on init) and configure beacon filering
only after bss_conf->dtimper is set (which means
mac80211 already parsed a beacon).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>