enic: Pass 802.1p bits for packets tagged with vlan zero

enic driver currently passes 802.1p bits to the upper layers for packets
tagged with non-zero vlan ids only. This patch extends such behaviour to
zero vlan tagged packets also.

The patch is dependant on the following kernel patches:

1) vlan_dev: VLAN 0 should be treated as "no vlan tag" (802.1p packet)
   - net-next-2.6 git commit: ad1afb00
   - Available 2.6.36 and later
2) vlan: Centralize handling of hardware acceleration.
  - net-next-2.6 git commit: 3701e513
  - Available 2.6.37 and later

Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Danny Guo <dannguo@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: David Wang <dwang2@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>