net: dsa: mv88e6xxx: remove basic function flags

All 88E6xxx Marvell switches (even the old not supported yet 88E6060)
have at least an ATU, per-port STP states and VLAN map, to run basic
switch functions such as Spanning Tree and port based VLANs.

Get rid of the related MV88E6XXX_FLAG_{ATU,PORTSTATE,VLANTABLE} flags,
as they are defaults to every chip.

This enables STP on 6185 and removes many inconsistencies on others.

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>