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

Introduced by commit 59835a (usb: gadget: multi: use
function framework for ACM.)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>