rp2/machine_uart: Fix event wait in uart.flush() and uart.read().

Do not wait in the worst case up to the timeout.

Fixes issue #13377.

Signed-off-by: robert-hh <robert@hammelrath.com>

ports/rp2/machine_uart.c

@@ -483,7 +483,7 @@ STATIC mp_uint_t mp_machine_uart_read(mp_obj_t self_in, void *buf_in, mp_uint_t
                     return i;
                 }
             }
-            mp_event_wait_ms(timeout - elapsed);
+            mp_event_handle_nowait();
         }
         *dest++ = ringbuf_get(&(self->read_buffer));
         start = mp_hal_ticks_ms(); // Inter-character timeout
@@ -559,7 +559,7 @@ STATIC mp_uint_t mp_machine_uart_ioctl(mp_obj_t self_in, mp_uint_t request, uint
             if (now >= timeout) {
                 break;
             }
-            mp_event_wait_ms((timeout - now) / 1000);
+            mp_event_handle_nowait();
         }
         *errcode = MP_ETIMEDOUT;
         ret = MP_STREAM_ERROR;