mac80211: make ieee80211_assign_beacon() static

This function is not used anywhere else than in cfg.c, so there's no
need to export it.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>