iwlwifi: mvm: assign quota per virtual interface

Instead of assigning quota per used binding (channel) assign
the same amount of quota for each virtual interface so that
when there are more than two interfaces using more than one
channel, we'll stay on the channels proportionally to the
number of virtual interfaces using the channels.

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