usb: gadget: multi: fix error return code in rndis_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 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>

drivers/usb/gadget/multi.c

@@ -157,8 +157,10 @@ static __init int rndis_do_config(struct usb_configuration *c)
 		return ret;
 
 	f_acm_rndis = usb_get_function(fi_acm);
-	if (IS_ERR(f_acm_rndis))
+	if (IS_ERR(f_acm_rndis)) {
+		ret = PTR_ERR(f_acm_rndis);
 		goto err_func_acm;
+	}
 
 	ret = usb_add_function(c, f_acm_rndis);
 	if (ret)