omap: usb: fix build warning

Fix this and similar build warnings when building with
omap_4430sdp_defconfig.

 CC      arch/arm/mach-omap2/board-4430sdp.o
In file included from arch/arm/mach-omap2/board-4430sdp.c:36:
arch/arm/plat-omap/include/plat/usb.h:109: warning: return type defaults to 'int'

Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>