net: forcedeth: fix compile warning of not used nv_set_tso function

Fix the below compile warning:
drivers/net/forcedeth.c:4266: warning: ‘nv_set_tso’ defined but not used

commit 569e1463 converts forcedeth driver to use hw_features.
So, implement function of .set_tso is abandoned.

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