ethtool: Remove redundant ret assignments

The assignment here will be overwritten, so it should be deleted

The clang_analyzer complains as follows:

net/ethtool/netlink.c:

Value stored to 'ret' is never read

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>