net: lan78xx: update for phy_(read|write)_mmd_indirect() removal

lan78xx appears to use phylib in a rather weird way, accessing the PHY
partly through phylib, and partly by making direct accesses to it,
including to the Clause 45 registers.  As the indirect MMD accessors are
going away, update this driver to use the plain phy_(read|write)_mmd()
accessors instead.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>