net: dsa: mv88e6xxx: rename PHY PPU functions

Respect the implicit naming convention used in all register sets
specific files, by renaming the mv88e6xxx_ppu_* functions with the
mv88e6xxx_phy_* prefix.

This is simply a s/xxx_ppu/xxx_phy_ppu/ substitution.

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