Correct SerialUSB return value

[sandeepmistry]

Resolves #138.

As mentioned in #138 SerialUSB.write would always return zero, this corrects the return value to return the number of bytes written.

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b79_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Correct SerialUSB return value #143
5. Select one of the boards under SAMD Pull Request Correct SerialUSB return value #143 in Tools->Board menu
6. Compile/Upload as usual