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

Introduced by commit cf9a08ae
(usb: gadget: convert source sink and loopback to new function interface)
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>