tg3: fix VLAN tagging regression

commit 92cd3a17ce9c719abb4c28dee3438e0c641f8de4
    tg3: Simplify tx bd assignments

broke VLAN tagging on outbound packets.
It ifdef'ed BCM_KERNEL_SUPPORTS_8021Q, but this
is not set anywhere. So vlan never gets set, and
all packets are sent with vlan=0.

v2: We can just remove the test. vlan_tx_tag_present
is valid regardless of whether the 802.1q module
is built.

Tested on BCM5721 rev 11.

Signed-off-by: Kasper Pedersen <kernel@kasperkp.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>