net: dsa: mv88e6xxx: use the new PHY API

This commit replaces every MDIO direct or indirect access with the new
generic mv88e6xxx_phy_* routines.

This allows us to get rid of the mv88e6xxx_mdio_{read,write}_{,in}direct
and {_,}mv88e6xxx_mdio_page_{read,write} functions.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>