net: vlan: enable soft features regardless of underlying device

If gso/gro feature of underlying device is turned off，
then new created vlan device never can turn gso/gro on. 

Although underlying device don't support TSO, we still
should use software segments for vlan device.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>