net/fm: Make the return value logic consistent with convention

In convention, the '0' is a normal return value indicating there isn't
an error. While some functions of FMan IM driver treat '0' as an error
return value.

Signed-off-by: Hou Zhiqiang <B48286@freescale.com>
Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>