SPL: fat: Fix spl_parse_image_header() return value handling

The spl_parse_image_header() can return 0 and it is not an error.
Only treat non-zero return value as an error.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Peng Fan <van.freenix@gmail.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>