iwlwifi: mvm: Add number of DTIMs to skip

New host-device API provides the ability to set the number
of DTIMs to skip. Add this parameter to the command and set
it (to a sane default value.)

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