USB: omap_udc: fix omap_udc_start() on 15xx machines

On OMAP 15xx machines there are no transceivers, and omap_udc_start()
always fails as it forgot to adjust the default return value.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>