usb: gadget: r8a66597-udc: fix error return code in r8a66597_probe()

Fix to return -ENOMEM in the request alloc error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>