mac80211: flush workqueue before calling driver ->stop() method

Since commit "mwl8k: handle station database update for AP's sta entry
via ->sta_notify()", mwl8k every now and then gets a command timeout
when ifconfig'ing a STA interface down.  This turns out to be due to
mwl8k_stop() being called while the work queue item that was scheduled
by mwl8k_sta_notify() to remove the STA entry for the associated AP is
still queued, and the former disables interrupts so that when the
latter eventually runs, a command completion interrupt is never seen.

Fix this by changing ieee80211_stop_device() so that the workqueue is
flushed before drv_stop() is called, instead of doing it the other way
around as is done now.  (As ->stop() is allowed to sleep, there isn't
any reason for drivers to queue work from within it.)

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>