usb: udc: store usb_udc pointer in struct usb_gadget

Instead of iterate to find usb_udc according to usb_gadget, this
way is easier.

Alan Stern suggests this way too:
http://marc.info/?l=linux-usb&m=142168496528894&w=2

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/include/linux/usb/gadget.h b/include/linux/usb/gadget.h
index 02476b3..242c694 100644
--- a/include/linux/usb/gadget.h
+++ b/include/linux/usb/gadget.h
@@ -474,6 +474,7 @@
 
 struct usb_gadget;
 struct usb_gadget_driver;
+struct usb_udc;
 
 /* the rest of the api to the controller hardware: device operations,
  * which don't involve endpoints (or i/o).
@@ -496,6 +497,7 @@
 /**
  * struct usb_gadget - represents a usb slave device
  * @work: (internal use) Workqueue to be used for sysfs_notify()
+ * @udc: struct usb_udc pointer for this gadget
  * @ops: Function pointers used to access hardware-specific operations.
  * @ep0: Endpoint zero, used when reading or writing responses to
  *	driver setup() requests
@@ -545,6 +547,7 @@
  */
 struct usb_gadget {
 	struct work_struct		work;
+	struct usb_udc			*udc;
 	/* readonly to gadget driver */
 	const struct usb_gadget_ops	*ops;
 	struct usb_ep			*ep0;