iwlwifi: mvm: tell firmware to let multicast frames in

Without this command, the firmware will filter out all the
multicast frames. Let them all in as for now. Later we will
want to optimize this to save power.

Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>