i40e/i40evf: Set Ethernet protocol correctly when Tx VLAN offloads are disabled

If transmit VLAN HW offloads are disabled then the network stack sends up
an skb with the protocol set to 8021q. In that case to get the correct
checksum offloads we have to reset the skb protocol to the encapsulated
ethertype.

Change-ID: I903d78533de09b1c5d3ec695ee1990dd0fa5dd0d
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>