usb: musb: omap2430: fix sparse warning

Fix the following sparse warning:

drivers/usb/musb/omap2430.c:54:33: warning: symbol '_glue' was not \
	declared. Should it be static?
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/musb/omap2430.c

@@ -51,7 +51,7 @@ struct omap2430_glue {
 };
 #define glue_to_musb(g)		platform_get_drvdata(g->musb)
 
-struct omap2430_glue		*_glue;
+static struct omap2430_glue	*_glue;
 
 static struct timer_list musb_idle_timer;