mac80211: remove bogus rcu_read_lock()

Another remnant of the previous key locking scheme
needs to be removed -- this causes a warning
otherwise as ieee80211_set_default_mgmt_key will
acquire a mutex.

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