In current i40e codebase, if single VLAN header is added in a packet,
it's treated as inner VLAN. Generally, a single VLAN header is
treated as the outer VLAN header, so update the driver behaviour
appropriately.
Fixes: 19b16e2f6442 ("ethdev: add vlan type when setting ether type")
Signed-off-by: Beilei Xing <beilei.xing@intel.com>