SAMD21: Increase UART buffer size to 256 bytes (from 64 bytes)

The upstream [ArduinoCore-samd](https://github.com/arduino/ArduinoCore-samd/) increased the UART buffer size in https://github.com/arduino/ArduinoCore-samd/commit/da9bfe8cf9aa92dc0e4f480be44fcecd5efe1604 to 256 bytes from 64 bytes (and later fixed a regression in https://github.com/arduino/ArduinoCore-samd/pull/651).

Would it be possible to get this change (or an equivalent) ported over?