tg3: Allow for recieve of full-size 8021AD frames

When receiving a vlan-tagged frame that still contains
a vlan header, the length of the packet will be greater
then MTU+ETH_HLEN since it will account of the extra
vlan header.  TG3 checks this for the case for 802.1Q,
but not for 802.1ad.  As a result, full sized 802.1ad
frames get dropped by the card.

Add a check for 802.1ad protocol when receving full
sized frames.

Suggested-by: Prashant Sreedharan <prashant@broadcom.com>
CC: Prashant Sreedharan <prashant@broadcom.com>
CC: Michael Chan <mchan@broadcom.com>
Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>