nrf/mpconfigport: Call tud_task() in MICROPY_EVENT_POLL_HOOK.

So that the interrupt character can interrupt a long-running loop, like a
sleep.

ports/nrf/mpconfigport.h

@@ -346,8 +346,15 @@ typedef long mp_off_t;
     /* micro:bit root pointers */ \
     void *async_data[2]; \
 
+#if MICROPY_HW_USB_CDC
+#define MICROPY_HW_USBDEV_TASK_HOOK extern void tud_task(void); tud_task();
+#else
+#define MICROPY_HW_USBDEV_TASK_HOOK ;
+#endif
+
 #define MICROPY_EVENT_POLL_HOOK \
     do { \
+        MICROPY_HW_USBDEV_TASK_HOOK \
         extern void mp_handle_pending(bool); \
         mp_handle_pending(true); \
         __WFI(); \