iwlwifi: mvm: don't require WOWLAN images when unified

WOWLAN images in unified firmwares should not be used, so don't require
them to support wowlan. This will allow to reduce the firmware's file
size.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>