rp2/machine_uart: Fix poll ioctl to also check hardware FIFO.

The RX IRQ does not trigger if the FIFO is less than the trigger level, in
which case characters may be available in the FIFO, yet not in the ringbuf,
and the ioctl returns false.

ports/rp2/machine_uart.c

@@ -477,7 +477,7 @@ STATIC mp_uint_t machine_uart_ioctl(mp_obj_t self_in, mp_uint_t request, mp_uint
     if (request == MP_STREAM_POLL) {
         uintptr_t flags = arg;
         ret = 0;
-        if ((flags & MP_STREAM_POLL_RD) && ringbuf_avail(&self->read_buffer) > 0) {
+        if ((flags & MP_STREAM_POLL_RD) && (uart_is_readable(self->uart) || ringbuf_avail(&self->read_buffer) > 0)) {
             ret |= MP_STREAM_POLL_RD;
         }
         if ((flags & MP_STREAM_POLL_WR) && ringbuf_free(&self->write_buffer) > 0) {