net: dsa: mv88e6xxx: add max VID to info

Some chips don't have a VLAN Table Unit, most of them do have a 4K
table, some others as the 88E6390 family has a 13th bit for the VID.

Add a new max_vid member to the info structure, used to check the
presence of a VTU as well as the value used to iterate from in VTU
GetNext operations.

This makes the MV88E6XXX_FLAG_VTU obsolete, thus remove it.

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>