mac80211: fix sdata locking around __ieee80211_request_smps

My cfg80211/mac80211 locking unification broke the sdata
locking in ieee80211_set_power_mgmt, it needs to acquire
the lock for __ieee80211_request_smps(). Add the locking.

Reported-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>