waveform.go: Refactored if block which calculates scaling factor in New.

Just thought I'd make a slight change. it's idiomatic to have a switch statement in this situation where there are multiple if statements, especially since only one variable is involved. I also made sure tests passed before submitting this pull request.

Since the resuting code that could be executed on each condition is only one line, I didn't think to place parentheses over the single statements, but let me know if that's what you prefer and I'll make the changes accordingly.