cfg80211: process events caused by suspend before suspending

When suspending without WoWLAN, cfg80211 will ask drivers to
disconnect. Even when the driver does this synchronously, and
immediately returns with a notification, cfg80211 schedules
the handling thereof to a workqueue, and may then call back
into the driver when the driver was already suspended/ing.

Fix this by processing all events caused by cfg80211_leave_all()
directly after that function returns. The driver still needs to
do the right thing here and wait for the firmware response, but
that is - at least - true for mwifiex where this occurred.

Reported-by: Amitkumar Karwar <akarwar@marvell.com>
Tested-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>