net: dsa: add support for ethtool get_rmon_stats()

Add support to allow dsa drivers to specify the .get_rmon_stats()
operation.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>