rockchip: mkimage: Use an existing macro instead of a decimal value

Depending on the SoC, a header of four characters is prepended to the
image. There is already a macro defining the number of characters:
RK_SPL_HDR_SIZE, so use it instead of hardcoding "4".

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/tools/rkcommon.c b/tools/rkcommon.c
index c2382df..8f281f5 100644
--- a/tools/rkcommon.c
+++ b/tools/rkcommon.c
@@ -307,11 +307,13 @@
 					   rkcommon_offset_to_spi(hdr1_offset));
 
 	for (i = 0; i < ARRAY_SIZE(spl_infos); i++) {
-		if (!memcmp(&hdr1_sdmmc->magic, spl_infos[i].spl_hdr, 4)) {
+		if (!memcmp(&hdr1_sdmmc->magic, spl_infos[i].spl_hdr,
+			    RK_SPL_HDR_SIZE)) {
 			if (spl_info)
 				*spl_info = &spl_infos[i];
 			return IH_TYPE_RKSD;
-		} else if (!memcmp(&hdr1_spi->magic, spl_infos[i].spl_hdr, 4)) {
+		} else if (!memcmp(&hdr1_spi->magic, spl_infos[i].spl_hdr,
+				   RK_SPL_HDR_SIZE)) {
 			if (spl_info)
 				*spl_info = &spl_infos[i];
 			return IH_TYPE_RKSPI;