mac80211: use correct station flags lock

This code is modifying the station flags, and
as such should hold the flags lock so it can
do so atomically vs. other flags modifications
and readers. This issue was introduced when
this code was added in eccb8e8f, as it used
the wrong lock (thus not fixing the race that
was previously documented in a comment.)

Cc: stable@kernel.org [2.6.31+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>