net: phy: marvell: avoid configuring fiber page for SGMII-to-Copper

When in SGMII-to-Copper mode, the fiber page is used for the MAC facing
link, and does not require configuration of the fiber auto-negotiation
settings.  Avoid trying.

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