Input: zforce - simplify function return logic

The invoked function already returns zero on success or a negative
errno code so there is no need to open code the logic in the caller.

This also fixes the following make coccicheck warning:

end returns can be simplified and declaration on line 602 can be dropped

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Heiko Stuebner <heiko.stuebner@bq.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>