rp2/machine_uart: Handle and clear UART RX timeout IRQ.

The pico-sdk 1.3.0 update in 97a7cc24
introduced a change that broke RP2 Bluetooth UART, and possibly UART in
general, which stops working right after UART is initialized.  The commit
raspberrypi/pico-sdk@2622e9b enables the UART receive timeout (RTIM) IRQ,
which is asserted when the receive FIFO is not empty, and no more
characters are received for a period of time.

This commit makes sure the RTIM IRQ is handled and cleared in
uart_service_interrupt.

ports/rp2/machine_uart.c

@@ -126,7 +126,7 @@ STATIC void uart_fill_tx_fifo(machine_uart_obj_t *self) {
 }
 
 STATIC inline void uart_service_interrupt(machine_uart_obj_t *self) {
-    if (uart_get_hw(self->uart)->mis & UART_UARTMIS_RXMIS_BITS) { // rx interrupt?
+    if (uart_get_hw(self->uart)->mis & (UART_UARTMIS_RXMIS_BITS | UART_UARTMIS_RTMIS_BITS)) { // rx interrupt?
         // clear all interrupt bits but tx
         uart_get_hw(self->uart)->icr = UART_UARTICR_BITS & (~UART_UARTICR_TXIC_BITS);
         if (!self->read_lock) {