usb: gadget: factor out two helper functions from composite_bind()

This patch factors out two helper functions from composite_bind()
that is composite_dev_prepare() and its counterpart
composite_dev_cleanup().

This will be used by the configfs which requries a slightly different
bind/setup code because part of its configurations (i.e. config
descripts, cdev, …) are setup in advance and VID/PID and so one should
not be overwritten. Also the setup of ep0 endpoint can be delayed until
the UDC is assigned.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/include/linux/usb/composite.h b/include/linux/usb/composite.h
index 771de7a..bd6d857 100644
--- a/include/linux/usb/composite.h
+++ b/include/linux/usb/composite.h
@@ -323,7 +323,15 @@
 extern int usb_composite_probe(struct usb_composite_driver *driver);
 extern void usb_composite_unregister(struct usb_composite_driver *driver);
 extern void usb_composite_setup_continue(struct usb_composite_dev *cdev);
+extern int composite_dev_prepare(struct usb_composite_driver *composite,
+		struct usb_composite_dev *cdev);
+void composite_dev_cleanup(struct usb_composite_dev *cdev);
 
+static inline struct usb_composite_driver *to_cdriver(
+		struct usb_gadget_driver *gdrv)
+{
+	return container_of(gdrv, struct usb_composite_driver, gadget_driver);
+}
 
 /**
  * struct usb_composite_device - represents one composite usb gadget