bridge: Support STP on VLAN devices

VLAN devices have type IFT_L2VLAN, so the STP code mistakenly believed
they couldn't be used for STP. That's not the case, so add the
ITF_L2VLAN to the check.

Reviewed by: donner@
MFC after: 1 week
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D28857

(cherry picked from commit 711ed156b94562c3dcb2ee9c1b3f240f960a75d2)