usb: gadget: s3c2410_udc: fix error return code in s3c2410_udc_probe()

Fix to return a negative error code in the gpio_to_irq() error handling
case instead of 0, as done elsewhere in this function.

Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>