net: phy: marvell: use mii_bmcr_encode_fixed()

Make use of the newly introduced mii_bmcr_encode_fixed() to get the
BMCR value when setting loopback mode for the 88e1510.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>