wifi: mac80211: remove unnecessary synchronize_net()

The call to ieee80211_do_stop() right after will also do
synchronize_rcu() to ensure the SDATA_STATE_RUNNING bit
is cleared, so we don't need to synchronize_net() here.

Change-Id: Id9f9ffcf195002013e5d9fde288877d219780864
Signed-off-by: Johannes Berg <johannes.berg@intel.com>