Fix stdio_set_chars_available_callback for usb (raspberrypi#1649)

When you get a callback to tell you a character is available, you should
be able to call getchar_timeout_us, but it's not working for USB

Fixes raspberrypi#1603

Co-authored-by: Andrew Gordon <[email protected]>

Author: peterharperuk

src/rp2_common/pico_stdio_usb/stdio_usb.c

@@ -57,7 +57,13 @@ static int64_t timer_task(__unused alarm_id_t id, __unused void *user_data) {
 static void low_priority_worker_irq(void) {
     if (mutex_try_enter(&stdio_usb_mutex, NULL)) {
         tud_task();
+#if PICO_STDIO_USB_SUPPORT_CHARS_AVAILABLE_CALLBACK
+        uint32_t chars_avail = tud_cdc_available();
+#endif
         mutex_exit(&stdio_usb_mutex);
+#if PICO_STDIO_USB_SUPPORT_CHARS_AVAILABLE_CALLBACK
+        if (chars_avail && chars_available_callback) chars_available_callback(chars_available_param);
+#endif
     } else {
         // if the mutex is already owned, then we are in non IRQ code in this file.
         //
@@ -147,12 +153,6 @@ int stdio_usb_in_chars(char *buf, int length) {
 }
 
 #if PICO_STDIO_USB_SUPPORT_CHARS_AVAILABLE_CALLBACK
-void tud_cdc_rx_cb(__unused uint8_t itf) {
-    if (chars_available_callback) {
-        usbd_defer_func(chars_available_callback, chars_available_param, false);
-    }
-}
-
 void stdio_usb_set_chars_available_callback(void (*fn)(void*), void *param) {
     chars_available_callback = fn;
     chars_available_param = param;