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

[sandeepmistry]

The upstream ArduinoCore-samd increased the UART buffer size in arduino/ArduinoCore-samd@da9bfe8 to 256 bytes from 64 bytes (and later fixed a regression in arduino/ArduinoCore-samd#651).

Would it be possible to get this change (or an equivalent) ported over?

[edspark]

Thanks for bringing this to our attention. It has been fixed here.