net: dsa: mv88e6xxx: add STU capability

Some switch models have a STU (per VLAN port state database). Add a new
capability flag to switches info, instead of checking their family.

Also if the 6165 family has an STU, it must have a VTU, so add the
MV88E6XXX_FLAG_VTU to its family flags.

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>