mv88e6xxx_port_vlan_join checks whether the VTU already contains an
entry for the given vid (via mv88e6xxx_vtu_getnext), and if so, merely
changes the relevant .member[] element and loads the updated entry
into the VTU.

However, at least for the mv88e6250, the on-stack struct
mv88e6xxx_vtu_entry vlan never has its .state[] array explicitly
initialized, neither in mv88e6xxx_port_vlan_join() nor inside the
getnext implementation. So the new entry has random garbage for the
STU bits, breaking VLAN filtering.

When the VTU entry is initially created, those bits are all zero, and
we should make sure to keep them that way when the entry is updated.

Fixes: 92307069 (net: dsa: mv88e6xxx: Avoid VTU corruption on 6097)
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Tested-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>