The pico-sdk 1.3.0 update in 97a7cc243b028833bdcb8ce0bc19b2bce7545851
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.
f21c565583