i40e: fix VLAN.TCI == 0 RX HW offload

This fixes two bugs in hardware VLAN offload:
 1. VLAN.TCI == 0 was being dropped
 2. there was a race between disabling of VLAN RX feature in hardware
    and processing RX queue, where packets processed in this window
    could have their VLAN information dropped

Fix moves the VLAN handling into i40e_process_skb_fields() to save on
duplicated code. i40e_receive_skb() becomes trivial and so is removed.

Signed-off-by: Michał Mirosław <michal.miroslaw@atendesoftware.pl>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>