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

sys/net/bridgestp.c

@@ -2232,6 +2232,7 @@ bstp_enable(struct bstp_port *bp)
 
 	switch (ifp->if_type) {
 		case IFT_ETHER:	/* These can do spanning tree. */
+		case IFT_L2VLAN:
 			break;
 		default:
 			/* Nothing else can. */