usb: gadget: dummy_hcd: fix error return code in init()

Fix to return -ENOMEM in the kzalloc() error handling case instead
of 0(following platform_device_add_data() will overwrite it to 0), as
done elsewhere in this function.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>