add usb_dc_init declare in usb_dc.h,and add usbd_initialize to call it

common/usb_dc.h

@@ -89,6 +89,11 @@ struct usbd_endpoint_cfg {
  * @{
  */
 
+/**
+ * @brief init device controller registers.
+ * @return 0 on success, negative errno code on fail.
+ */
+int usb_dc_init(void);
 /**
  * @brief Set USB device address
  *
@@ -172,10 +177,11 @@ int usbd_ep_write(const uint8_t ep, const uint8_t *data, uint32_t data_len, uint
 /**
  * @brief Read data from the specified endpoint
  *
- * This is similar to usb_dc_ep_read, the difference being that, it doesn't
- * clear the endpoint NAKs so that the consumer is not bogged down by further
- * upcalls till he is done with the processing of the data. The caller should
- * reactivate ep by setting max_data_len 0 do so.
+ * This function is called by the endpoint handler function, after an OUT
+ * interrupt has been received for that EP. The application must only call this
+ * function through the supplied usbd_ep_callback function. This function clears
+ * the ENDPOINT NAK when max_data_len is 0, if all data in the endpoint FIFO has been read,
+ * so as to accept more data from host.
  *
  * @param[in]  ep           Endpoint address corresponding to the one
  *                          listed in the device configuration table

core/usbd_core.c

@@ -1297,3 +1297,9 @@ bool usb_device_is_configured(void)
 {
     return usbd_core_cfg.configured;
 }
+
+int usbd_initialize(void)
+{
+    usb_dc_init();
+    return 0;
+}

core/usbd_core.h

@@ -131,6 +131,8 @@ void usbd_class_add_interface(usbd_class_t *devclass, usbd_interface_t *intf);
 void usbd_interface_add_endpoint(usbd_interface_t *intf, usbd_endpoint_t *ep);
 bool usb_device_is_configured(void);
 
+int usbd_initialize(void);
+
 #ifdef __cplusplus
 }
 #endif