isdn: fix obvious cut-and-paste error in st5481_usb.c

Fix a rather obvious cut-and-paste error, where earlier code for the
controller URB got somehow mixed in with code for the interrupt URB.
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/isdn/hisax/st5481_usb.c

@@ -342,7 +342,7 @@ void st5481_release_usb(struct st5481_adapter *adapter)
 	usb_kill_urb(intr->urb);
 	kfree(intr->urb->transfer_buffer);
 	usb_free_urb(intr->urb);
-	ctrl->urb = NULL;
+	intr->urb = NULL;
 }
 
 /*