USB CDC Serial_::begin fixes.

On a board with native USB (e.g. Leonardo), Serial.begin(115200); gives a compiler warning:
   warning: large integer implicitly truncated to unsigned type
Serial.begin(9600, SERIAL_8N1); doesn't compile at all.

Pull request fixes these by:
- Change the CDC Serial_::begin prototype to match that of HardwareSerial::begin.
- Two argument form of begin()