mac80211: flush workqueue a second time in suspend()

Drivers can theoretically queue more work in one of their callbacks
from mac80211 suspend, so let's flush it once more to be on the safe
side, just before calling ->stop().

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>