iwlwifi: mvm: Add RX statistics debugfs entry

Add a debugfs entry for the RX statistics received from
the firmware.

Signed-off-by: Matti Gottlieb <matti.gottlieb@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>