mac80211: add suspend/resume callbacks

This patch introduces suspend and resume callbacks to mac80211,
allowing mac80211 to quiesce its state (bringing down interfaces,
removing keys, etc) in preparation for suspend.  cfg80211 will call
the suspend hook before the device suspend, and resume hook after
the device resume.

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