ethtool: fix ->reply_size() error handling

The "ret < 0" comparison is never true because "ret" is still zero.

Fixes: 728480f1 ("ethtool: default handlers for GET requests")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>