selftests: forwarding: ethtool_mm: fall back to aggregate if device does not report pMAC stats

Some devices do not support individual 'pmac' and 'emac' stats.
For such devices, resort to 'aggregate' stats.

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>