add usb_background_schedule()

unconditionally schedule usb background after
background_callback_reset()

main.c

@@ -184,6 +184,7 @@ STATIC void stop_mp(void) {
     #endif
 
     background_callback_reset();
+    usb_background_schedule();
 
     gc_deinit();
 }

supervisor/shared/background_callback.c

@@ -105,9 +105,6 @@ void background_callback_end_critical_section() {
     CALLBACK_CRITICAL_END;
 }
 
-extern background_callback_t usb_callback;
-extern void usb_background_do(void* unused);
-
 void background_callback_reset() {
     CALLBACK_CRITICAL_BEGIN;
     background_callback_t *cb = (background_callback_t*)callback_head;
@@ -120,8 +117,6 @@ void background_callback_reset() {
     callback_tail = NULL;
     in_background_callback = false;
     CALLBACK_CRITICAL_END;
-
-    background_callback_add(&usb_callback, usb_background_do, NULL);
 }
 
 void background_callback_gc_collect(void) {

supervisor/shared/usb/usb.c

@@ -93,16 +93,21 @@ void usb_background(void) {
     }
 }
 
-/*static*/ background_callback_t usb_callback;
+static background_callback_t usb_callback;
-/*static*/ void usb_background_do(void* unused) {
+static void usb_background_do(void* unused) {
     usb_background();
 }
 
-void usb_irq_handler(void) {
+void usb_background_schedule(void)
-    tud_int_handler(0); \
+{
     background_callback_add(&usb_callback, usb_background_do, NULL);
 }
 
+void usb_irq_handler(void) {
+    tud_int_handler(0);
+    usb_background_schedule();
+}
+
 //--------------------------------------------------------------------+
 // tinyusb callbacks
 //--------------------------------------------------------------------+

supervisor/usb.h

@@ -35,6 +35,9 @@
 // it may be necessary to call it directly.
 void usb_background(void);
 
+// Schedule usb background
+void usb_background_schedule(void);
+
 // Ports must call this from their particular USB IRQ handler
 void usb_irq_handler(void);