mac80211: fix channel context iteration

During suspend/resume channel contexts might be
iterated even if they haven't been re-added to
the driver, keep track of this and skip them in
iteration. Also use the new status for sanity
checks.

Also clarify the fact that during HW restart all
contexts are iterated over (thanks Eliad.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>