spl: spi: Do not hardcode fixed size for header

Find out size directly from header structure.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

common/spl/spl_spi.c

@@ -28,7 +28,7 @@ static int spi_load_image_os(struct spl_image_info *spl_image,
 	int err;
 
 	/* Read for a header, parse or error out. */
-	spi_flash_read(flash, CONFIG_SYS_SPI_KERNEL_OFFS, 0x40,
+	spi_flash_read(flash, CONFIG_SYS_SPI_KERNEL_OFFS, sizeof(*header),
 		       (void *)header);
 
 	if (image_get_magic(header) != IH_MAGIC)
@@ -88,7 +88,7 @@ static int spl_spi_load_image(struct spl_image_info *spl_image,
 		return -ENODEV;
 	}
 
-	header = spl_get_load_buffer(-sizeof(*header), 0x40);
+	header = spl_get_load_buffer(-sizeof(*header), sizeof(*header));
 
 #if CONFIG_IS_ENABLED(OF_CONTROL) && !CONFIG_IS_ENABLED(OF_PLATDATA)
 	payload_offs = fdtdec_get_config_int(gd->fdt_blob,
@@ -101,7 +101,7 @@ static int spl_spi_load_image(struct spl_image_info *spl_image,
 #endif
 	{
 		/* Load u-boot, mkimage header is 64 bytes. */
-		err = spi_flash_read(flash, payload_offs, 0x40,
+		err = spi_flash_read(flash, payload_offs, sizeof(*header),
 				     (void *)header);
 		if (err) {
 			debug("%s: Failed to read from SPI flash (err=%d)\n",