The recent change bringing device-tree support triggers a warning in a
non device-tree build :
  drivers/usb/gadget/udc/pxa27x_udc.c:2405:28: warning: 'udc_pxa_dt_ids'
  defined but not used [-Wunused-variable]

Fix the warning with a preprocessor condition.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>