iwlwifi: mvm: Fix quota handling for monitor interface

1. Quota for the monitor interface should be added only if there is
   a channel context assigned to the interface.
2. In the unassign channel context flow, need to remove the quota
   for the monitor interface binding, before unbinding.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>