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>