mac80211: allow cancelling dependent ROCs

In my redesign of remain-on-channel I forgot
that an item could be cancelled when it's a
dependent item that is part of another item.
Allow cancelling such items by removing them
from the dependents list.

Note that when we cancel the main item, all
its dependents are also cancelled. It would
be possible to not do that, but would need
tricks to promote an item from dependent to
top-level and is tricky in the HW ROC case.

Reported-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>