cfg80211: add PM hooks

This should help implement suspend/resume in mac80211, these
hooks will be run before the device is suspended and after it
resumes. Therefore, they can touch the hardware as much as
they want to.

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