[usb] fix user_callback_set_config

USB using control callbacks was broken since d6bad27735
Properly check if user usb control callback are registered,
while here make sure user_callback_set_config callback pointers are initialized to NULL.

lib/usb/usb.c

@@ -86,6 +86,11 @@ usbd_device *usbd_init(const usbd_driver *driver,
 	usbd_dev->user_callback_ctr[0][USB_TRANSACTION_IN] =
 	    _usbd_control_in;
 
+	int i;
+	for (i = 0; i < MAX_USER_SET_CONFIG_CALLBACK; i++) {
+		usbd_dev->user_callback_set_config[i] = NULL;
+	}
+
 	return usbd_dev;
 }