net: phy: mscc: add ethtool statistics counters

There are a few counters available in the PHY: receive errors, false
carriers, link disconnects, media CRC errors and valids counters.

So let's expose those in the PHY driver.

Use the priv structure as the next PHY to be supported has a few
additional counters.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microsemi.com>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>