[SCSI] cxgb3i: fix connection error when vlan is enabled

There is a bug when VLAN is configured on the cxgb3 interface, the iscsi
conn. would be denied with message "cxgb3i: NOT going through cxgbi device."

This patch adds code to get the real egress net_device when vlan is configured.

Signed-off-by: Karen Xie <kxie@chelsio.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>