usb: wusbcore: fix panic in wusbhc_chid_set

If no valid CHID value has previously been set on an HWA, writing a
value of all zeros will cause a kernel panic in uwb_radio_stop because
wusbhc->uwb_rc has not been set.  This patch skips the call to
uwb_radio_stop if wusbhc->uwb_rc has not been initialized.
Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/wusbcore/mmc.c

@@ -301,7 +301,7 @@ int wusbhc_chid_set(struct wusbhc *wusbhc, const struct wusb_ckhdid *chid)
 
 	if (chid)
 		result = uwb_radio_start(&wusbhc->pal);
-	else
+	else if (wusbhc->uwb_rc)
 		uwb_radio_stop(&wusbhc->pal);
 
 	return result;