mac80211: make workqueue freezable

This patch makes the mac80211 workqueue freezable making it
interact a bit better with system suspend and not try to ping
the AP while the hardware is down.

This doesn't really help with implementing proper suspend in
any way but makes some bad things trigger less.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>