Staging: panel: Prevent double-calling of parport_release - fix oops.

This patch prevents the code from calling parport_release and
parport_unregister_device twice with the same arguments - and thus fixes an oops.

Rationale:
After the first call the parport is already released and the
handle isn't valid anymore and calling parport_release and
parport_unregister_device twice isn't a good idea.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Cc: stable <stable@kernel.org>
Acked-by: Willy Tarreau <w@1wt.eu>

drivers/staging/panel/panel.c

@@ -2277,6 +2277,7 @@ int panel_init(void)
 		if (pprt) {
 			parport_release(pprt);
 			parport_unregister_device(pprt);
+			pprt = NULL;
 		}
 		parport_unregister_driver(&panel_driver);
 		printk(KERN_ERR "Panel driver version " PANEL_VERSION
@@ -2327,6 +2328,7 @@ static void __exit panel_cleanup_module(void)
 		/* TODO: free all input signals */
 		parport_release(pprt);
 		parport_unregister_device(pprt);
+		pprt = NULL;
 	}
 	parport_unregister_driver(&panel_driver);
 }