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>