gianfar: fix bug caused by 87c288c6

commit 87c288c6 "gianfar: do vlan cleanup" has two issues:
# permutation of rx and tx flags
# enabling vlan tag insertion by default (this leads to unusable connections on some configurations)

If VLAN insertion is requested (via ethtool) it will be set at an other point ...

Signed-off-by: Sebastian Poehn <sebastian.poehn@belden.com>
Signed-off-by: David S. Miller <davem@davemloft.net>