hw/char/max78000_uart: Destroy FIFO on deinit

hw/char/max78000_uart: Destroy FIFO on deinit

In the max78000_uart we create a FIFO in the instance_init function,
but we don't destroy it on deinit, so ASAN reports a leak in the
device-introspect-test:

    #0 0x561cc92d5de3 in malloc (/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/arm-asan/qemu-system-arm+0x21f1de3) (BuildId: 98fdf9fc85c3beaeca8eda0be8412f1e11b9c6ad)
    #1 0x70cbf2afab09 in g_malloc (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x62b09) (BuildId: 1eb6131419edb83b2178b682829a6913cf682d75)
    #2 0x561ccc4c884d in fifo8_create /mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/arm-asan/../../util/fifo8.c:27:18
    #3 0x561cc9744ec9 in max78000_uart_init /mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/arm-asan/../../hw/char/max78000_uart.c:241:5

Add an instance_finalize method to destroy the FIFO.

Cc: qemu-stable@nongnu.org
Fixes: d447e4b70295 ("MAX78000: UART Implementation")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250821154358.2417744-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>