Use TX buffer for Wire slave writes

[sandeepmistry]

Brings things inline with SAM core Wire lib. Also, allows for more consistent behaviour if the master requests more data than the slave writes.

The buffered TX data is:

• sent one byte at a time on DRDY IRQ
• cleared when master read mode is started/restarted
• 0xff is sent if a read is triggered and the TX buffer is empty

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b82_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 Use TX buffer for Wire slave writes #98
5. Select one of the boards under SAMD Pull Request Use TX buffer for Wire slave writes #98 in Tools->Board menu
6. Compile/Upload as usual