Change max baud to 230400

Only affects osx (see https://github.com/facchinm/go-serial/blob/v1/serial_darwin_amd64.go#L32)

Author: facchinm

src/github.com/arduino-libraries/WiFi101-FirmwareUpdater/nina/flasher.go

@@ -237,7 +237,7 @@ func (flasher *Flasher) Md5sum(data []byte) error {
 
 func OpenSerial(portName string) (serial.Port, error) {
 	mode := &serial.Mode{
-		BaudRate: 1000000,
+		BaudRate: 230400,
 		Vtimeout: 100,
 		Vmin: 0,
 	}