net: phy: marvell10g: clean up interface mode switching

Centralise the PHY interface mode switching, rather than having it in
two places.

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>