i40e: Fix VEB/VEPA bridge mode mismatch issue

Fix i40e_is_vsi_uplink_mode_veb to check if bridge is actually
in VEB mode before allowing LB in the add VSI routine, instead of
unconditionally returning VEB bridge mode.

Change-ID: I162397b1bdd02367735fe9baaeb51465be2a3ce9
Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>