usb: gadget: udc: atmel: Use dev_err() to display EP configuration error

Use dev_err() to display EP configuration error to avoid silent failure.

Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>