gofmt with tensorflow/go/genop/internal/genop.go (#17710)

By default golang use `gofmt -s` to format the code though
it looks like tensorflow/go/genop/internal/genop.go is not
formattted with gofmt.

This fix updates with gofmt.
https://goreportcard.com/report/github.com/tensorflow/tensorflow



Signed-off-by: Yong Tang <yong.tang.github@outlook.com>