cfg80211: mlme API must be able to sleep

After the mac80211 mlme cleanup, we can require that
the MLME functions in cfg80211 can sleep. This will
simplify future work in cfg80211 a lot.

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