drivers: net: phy: Correct duplicate MDIO_XGENE entry

An extra entry for MDIO_XGENE got added during merging.
Delete it.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>