i40e: disable buggy VEB switching

Disable VEB switching by removing ALLOW_LB on SRIOV vsi.

If the source mac address of packet sent from VF is not listed in the
VEB's mac table, the VEB will switch the packet back to the VF.
It's a hardware issue. Enabling ALLOW_LB flag will block VF functions.

Test report: http://www.dpdk.org/ml/archives/dev/2015-March/015687.html

Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>
Tested-by: Huilong Xu <huilongx.xu@intel.com>