usb: omap_control_usb: fix compile warning

When CONFIG_OMAP_CONTROL_USB isn't enabled,
there's a compile warning stating that a
particular function isn't a prototype.

Fix it.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>