Marvell phy: add field to get errors from fiber link.

Add support for the fiber receiver error counter in the
statistics. Rename the current counter which is for copper errors to
phy_receive_errors_copper, so it is easy to distinguish copper from
fiber.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@nexvision.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>