cxgbev(4): Pay attention to the VLAN configuration for the VF.

Make sure that the transmit traffic is tagged correctly or else the
firmware will refuse to transmit and will report an ACL violation.

On receive the hardware will make sure that tagged traffic is delivered
to the appropriate VM. The driver only asserts that the VLAN id that
was extracted from the wire traffic matches the VF's configuration.

All this works when associating a specific VLAN id with a VF. The
'trunk' setting likely needs more work.

MFC after: 1 week
Sponsored by: Chelsio Communications

(cherry picked from commit 2d0a01271223ce623c78f5c8236f8f3f4b6ef104)