ANDROID: usb: gadget: Do not disconnect unregistered dev

configfs_composite_unbind sets the gadget data
to null. Therefore, add check in disconnect
function to make sure that cdev is not NULL.
Prints a WARN message if the driver tries to
redundantly disconnect a gadget.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Change-Id: I248cb7175d0dd9a51c18053dd39475d8b3284f6d

drivers/usb/gadget/composite.c

@@ -1983,6 +1983,12 @@ void composite_disconnect(struct usb_gadget *gadget)
 	struct usb_composite_dev	*cdev = get_gadget_data(gadget);
 	unsigned long			flags;
 
+	if (cdev == NULL) {
+		WARN(1, "%s: Calling disconnect on a Gadget that is \
+			 not connected\n", __func__);
+		return;
+	}
+
 	/* REVISIT:  should we have config and device level
 	 * disconnect callbacks?
 	 */