net: dsa: mv88e6xxx: move VTU flush

Move the VTU flush operation to global1_vtu.c and call it from a
mv88e6xxx_vtu_setup helper, similarly to the ATU and PVT setup.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>