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>

common/spl/spl_fat.c

@@ -58,7 +58,7 @@ int spl_load_image_fat(struct blk_desc *block_dev,
 		goto end;
 
 	err = spl_parse_image_header(header);
-	if (err <= 0)
+	if (err)
 		goto end;
 
 	err = file_fat_read(filename, (u8 *)spl_image.load_addr, 0);