code quality

[petar-dambovaliev]

Please, fix these nested if-else statements.
Early returns are much easier to read.
An example of that would be in the control.go file with the error handling.
If there is no error, the length of the data is acceptable and everything is okay, it seems that this will always return an error.

if err == nil {
		if length := len(response.Data); length > 18 { // 18 is the minimum expected
			if int(response.Data[19]) != pduStart {
				err = fmt.Errorf(ErrorText(errCliCannotStartPLC))
			} else {
				if int(response.Data[20]) == pduAlreadyStarted {
					err = fmt.Errorf(ErrorText(errCliAlreadyRun))
				} else {
					err = fmt.Errorf(ErrorText(errCliCannotStartPLC))
				}
			}
		} else {
			err = fmt.Errorf(ErrorText(errIsoInvalidPDU))
		}
	}

Also, there are mistakes in the Stop method.
int(response.Data[20]) == pduAlreadyStarted
Should be
int(response.Data[20]) == pduAlreadyStopped