bridgestp: Ensure we send STP on VLAN interfaces

Reviewed by: donner@
MFC after: 1 week
X-MFC-with: 711ed156b94562c3dcb2ee9c1b3f240f960a75d2
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D28916

(cherry picked from commit f5537cd0693c85efdb2180a0a107c51eae15ba39)