net: dsa: mv88e6xxx: Ensure STU support in VLAN MSTI callback

In the same way that we check for STU support in the MST state
callback, we should also verify it before trying to change a VLANs
MSTI membership.

Fixes: acaf4d2e ("net: dsa: mv88e6xxx: MST Offloading")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>