Regression in UART buffer size

[sandeepmistry]

In the v1.8.9 release of the core the default ring buffer size was set to 256: https://github.com/arduino/ArduinoCore-samd/blob/1.8.9/cores/arduino/RingBuffer.h#L26-L32

After #560 was merged it looks like this is reduced to 64 bytes now as api/RingBuffer.h is used instead of RingBuffer.h in cores/arduino/SafeRingBuffer.h: https://github.com/arduino/ArduinoCore-API/blob/master/api/RingBuffer.h#L33

This might also be related to #644