net: dsa: mv88e6xxx: split VTU entry data member

VLAN aware Marvell chips can program 802.1Q VLAN membership as well as
802.1s per VLAN Spanning Tree state using the same 3 VTU Data registers.

Some chips such as 88E6185 use different Data registers offsets for
ports state and membership, and program them in a single operation.

Other chips such as 88E6352 use the same register layout but program
them in distinct operations (an indirect table is used for 802.1s.)

Newer chips such as 88E6390 use the same offsets for both state and
membership in distinct operations, thus require multiple data accesses.

To correctly abstract this, split the "data" structure member of
mv88e6xxx_vtu_entry in two "state" and "member" members, before adding
VTU support for newer chips.

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>