tg3: Do not include vlan acceleration features in vlan_features

Including hardware acceleration features in vlan_features breaks
stacked vlans (Q-in-Q) by marking the bottom vlan interface as
capable of acceleration.  This causes one of the tags to be lost
and the packets are sent with a sing vlan header.

CC: Nithin Nayak Sujir <nsujir@broadcom.com>
CC: Michael Chan <mchan@broadcom.com>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>