USB: otg: fix link breakage, when the NOP USB Xceiver is a module

If the NOP USB OTG transceiver driver is built as a module, the otg.h
header declares external functions, but if they are referenced from the
kernel proper, as, e.g., in the OMAP3 case, where the omap3evm board is
calling the usb_nop_xceiv_register() function, linkage breaks. This patch
fixes this problem.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>