mac80211: don't call conf_tx under RCU lock

Reinette pointed out that with the sta_info RCU-ification
the behaviour here changed and the conf_tx callback is
now invoked under RCU read lock. That is not necessary so
this patch restores the original behaviour

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>