rp2/machine_uart: Fix read when FIFO has chars but ringbuf doesn't.
rp2/machine_uart: Fix read when FIFO has chars but ringbuf doesn't.

Prior to this fix, if the UART hardware FIFO had a few chars but still
below the FIFO trigger threshold, and the ringbuf was empty, the read
function would timeout if timeout==0 (the default timeout).

This fix follows the suggestion of @iabdalkader.

adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython