USB-CDC: ethernet error path potential oops fix

Fix potential oops on rare error path.
The patch is based on commit e7b13ec9235b9fded90f826ceeb8c34548631351
(done by David Brownell <david-b@pacbell.net>) from linux-2.6.git.

Description of the issue taken from linux kernel bugzilla:
(https://bugzilla.kernel.org/show_bug.cgi?id=9594)

The potential error can be tracked down as follows:

(1) line 807: let the second conjunct on the "if" statment be false
    meaning "dev->status_ep" is null. This means the "if" evaluates
    to false.

follow thru the code until...

(2) line 808: usb_ep_disable(dev->status_ep) passes in a null argument,
    however "usb_ep_disable" cannot handle that:

(from include/linux/usb/gadget.h)
191 static inline int
192 usb_ep_disable (struct usb_ep *ep)
193 {
194         return ep->ops->disable (ep);
195 }

--

Signed-off-by: Vitaly Kuzmichev <vkuzmichev@mvista.com>

drivers/usb/gadget/ether.c

@@ -801,7 +801,7 @@ done:
 
 	/* on error, disable any endpoints  */
 	if (result < 0) {
-		if (!subset_active(dev))
+		if (!subset_active(dev) && dev->status_ep)
 			(void) usb_ep_disable (dev->status_ep);
 		dev->status = NULL;
 		(void) usb_ep_disable (dev->in_ep);