net: dsa: Pass VLAN MSTI migration notifications to driver

Add the usual trampoline functionality from the generic DSA layer down
to the drivers for VLAN MSTI migrations.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>