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