usb: gadget: cdc2: fix error return code in cdc_do_config()

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in this function.

Introduced by commit 29a664 (usb: gadget: cdc2: use function
framework for ACM)

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