gianfar: fix default tx vlan offload feature flag

Commit -
"b852b720 gianfar: fix bug caused by
87c288c6"
disables by default (on mac init) the hw vlan tag insertion.
The "features" flags were not updated to reflect this, and
"ethtool -K" shows tx-vlan-offload to be "on" by default.

Cc: Sebastian Poehn <sebastian.poehn@belden.com>
Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>