iwlwifi: mvm: Change power management dependency on multi MAC

FW still does not support power management on multiple MAC interfaces.
Currently the driver enforce this limitation by disabling PM if second
interface is added. Change this behavior to allow PM on a single interface
even if other interfaces exist but not bound to any specific PHY.
PM will be enabled if only one single interface is bound.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>