mlx4_core: Add VLAN tag field to WQE control segment struct

Add fields for VLAN tag and insert VLAN tag flag to the control
section struct.  These fields will be used for sending ethernet
packets.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>