iwlwifi: mvm: make RSS RX more robust

If the firmware ever decides to send any new/more notifications
to the RSS queues, the driver would currently try to interpret
those as REPLY_RX_MPDU_CMD and, if the notification was small,
access invalid memory.

Prevent that by checking for REPLY_RX_MPDU_CMD explicitly which
allows ignoring unexpected notifications.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>