Input: zforce - fix error return code in zforce_start()

The error code was not set if unable to set config, so the error
condition wasn't reflected in the return value. Fix to return a
negative error code from the error handling case instead of 0.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>