net: dsa: mv88e6xxx: support the VTU Page bit

Newer chips such as the 88E6390 have a VTU Page bit in the VTU VID
register to specify a 13th bit for the VID. This can be used to support
8K VLANs.

When dumping the whole VTU, all VID bits must be set to one, including
this VTU Page bit. Add support for VID greater than 4095.

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