mac80211: use station mutex in configuration

mac80211: use station mutex in configuration

There's no need to use RCU here, we can just lock
the station mutex instead. This allows the code
to sleep, which is necessary for later patches.

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