net: phy: smsc: Implement PHY statistics

Most of the PHYs supported by the SMSC driver have a counter of symbol
errors. This is 16 bit wide and wraps around when it reaches its
maximum value.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-By: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>