USB: musb: omap2430: fix sparse warning

Fix the following sparse warning:

drivers/usb/musb/omap2430.c:314:16: warning: Using plain integer as NULL pointer
Signed-off-by: NFelipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/musb/omap2430.c

@@ -315,7 +315,7 @@ int musb_platform_exit(struct musb *musb)
 	musb_platform_suspend(musb);
 
 	clk_put(musb->clock);
-	musb->clock = 0;
+	musb->clock = NULL;
 
 	return 0;
 }