net: dsa: Provide additional RMON statistics

Display sw_in_discards, sw_in_filtered, and sw_out_filtered for chips
supported by mv88e6123_61_65 and mv88e6352 drivers.

The variables are provided in port registers, not the normal status registers.
Mark by adding 0x100 to the register offset and add special handling code
to mv88e6xxx_get_ethtool_stats.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>