mac80211: implement multi-vif in-place reservations

Multi-vif in-place reservations happen when
it is impossible to allocate more channel contexts
as indicated by interface combinations.

Such reservations are not finalized until all
assigned interfaces are ready.

This still doesn't handle all possible cases
(i.e. degradation of number of channels) properly.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>