usb: gadget: function: phonet: balance usb_ep_disable calls

commit 9ec36f7f upstream.

f_phonet's ->set_alt() method will call usb_ep_disable()
potentially on an endpoint which is already disabled. That's
something the gadget/function driver must guarantee that it's
always balanced.

In order to balance the calls, just make sure the endpoint
was enabled before by means of checking the validity of
driver_data.
Reported-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
[ luis: backported to 3.16:
  - file rename: drivers/usb/gadget/function/f_phonet.c ->
    drivers/usb/gadget/f_phonet.c ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/usb/gadget/f_phonet.c

@@ -417,7 +417,10 @@ static int pn_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
 			return -EINVAL;
 
 		spin_lock(&port->lock);
-		__pn_reset(f);
+
+		if (fp->in_ep->driver_data)
+			__pn_reset(f);
+
 		if (alt == 1) {
 			int i;