IB/mlx4: Don't set VLAN in IBoE WQEs' control segment

There's no need to set the vlan-related fields in an IBoE send WQE
control segment:

 - the vlan to be used by a UD QP is set in the datagram segment.
 - for GSI (CM) QP, all the headers down to 8021q and MAC are built by
   the software anyway.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>