ixgbe: Fix receive on real device when VLANs are configured

Traffic received with a priority tag (VID = 0) and non-zero priority value was
incorrectly handled by the VLAN packet code path due to a check on zero for
the whole VLAN tag instead of just the VID.

This patch masked out the priority field when checking the vlan tag for
received VLAN packets.

Signed-off-by: Lucy Liu <lucy.liu@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>