cfg80211: allow registering more than one beacon listener

The commit:

commit 5e760230
Author: Johannes Berg <johannes.berg@intel.com>
Date:   Fri Nov 4 11:18:17 2011 +0100

    cfg80211: allow registering to beacons

allowed only a single process to register for beacon events
per wiphy.  This breaks cases where a user may want two or
more VIFs on a wiphy and run a seperate hostapd process on
each vif.

This patch allows multiple beacon listeners, fixing the
regression.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>