common: spl: ram: fix return code

Instead of always retuning success, return actual result of
load_simple_fit_image or spl_parse_image_header, otherwise we
might end up jumping on uninitialized spl_image->entry_point.

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Stefan Roese <sr@denx.de>