net: dsa: mv88e6xxx: Avoid VTU corruption on 6097

As soon as you add the second port to a VLAN, all other port
membership configuration is overwritten with zeroes. The HW interprets
this as all ports being "unmodified members" of the VLAN.

In the simple case when all ports belong to the same VLAN, switching
will still work. But using multiple VLANs or trying to set multiple
ports as tagged members will not work.

On the 6352, doing a VTU GetNext op, followed by an STU GetNext op
will leave you with both the member- and state- data in the VTU/STU
data registers. But on the 6097 (which uses the same implementation),
the STU GetNext will override the information gathered from the VTU
GetNext.

Separate the two stages, parsing the result of the VTU GetNext before
doing the STU GetNext.

We opt to update the existing implementation for all applicable chips,
as opposed to creating a separate callback for 6097, because although
the previous implementation did work for (at least) 6352, the
datasheet does not mention the masking behavior.

Fixes: ef6fcea3

 ("net: dsa: mv88e6xxx: get STU entry on VTU GetNext")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Link: https://lore.kernel.org/r/20201112114335.27371-1-tobias@waldekranz.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>