net: dsa: mv88e6xxx: move VTU Data accessors

The code to access the VTU Data registers currently only supports the
88E6185 family and alike: 2-bit membership adjacent to 2-bit port state.

Even though the 88E6352 family introduced an indirect table to program
the VLAN Spanning Tree states, the usage of the VTU Data registers
remains the same regardless the VTU or STU operation.

Now that the mv88e6xxx_vtu_entry structure contains both port membership
and states data, factorize the code to access them in global1_vtu.c.

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>