cfg80211: export cfg80211_ref_bss

This is needed by mac80211 to keep a reference
to a BSS alive for the auth process. Remove the
old version of cfg80211_ref_bss() since it's
not actually used.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>