net: phy: marvell10g: read copper results from CSSR1

Read the copper autonegotiation results from the copper specific
status register, rather than decoding the advertisements. Reading
what the link is actually doing will allow us to support downshift
modes.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>