net: phy: micrel: use phy_read_mmd and phy_write_mmd

This driver implements open-coded versions of phy_read_mmd() and
phy_write_mmd() for KSZ9031. That's not needed, let's use the
phylib functions directly.

This is compile-tested only because I have no such hardware.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>