sunvnet: does not support GSO for sctp

The NETIF_F_GSO_SOFTWARE implies support for GSO on SCTP, but the
sunvnet driver does not support GSO for sctp.  Here we remove the
NETIF_F_GSO_SOFTWARE feature flag and only report NETIF_F_ALL_TSO
instead.

Signed-off-by: Cathy Zhou <Cathy.Zhou@Oracle.COM>
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>