xilinx: board: Use ETH_ALEN macro for mac address size

Use predefined macro for eth_mac legacy format.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/382d4bff370894164fad3c30ac0400b25142b544.1674573561.git.michal.simek@amd.com
diff --git a/board/xilinx/common/board.c b/board/xilinx/common/board.c
index 52a3814..6cdb000 100644
--- a/board/xilinx/common/board.c
+++ b/board/xilinx/common/board.c
@@ -86,7 +86,7 @@
 struct xilinx_legacy_format {
 	char board_sn[18]; /* 0x0 */
 	char unused0[14]; /* 0x12 */
-	char eth_mac[6]; /* 0x20 */
+	char eth_mac[ETH_ALEN]; /* 0x20 */
 	char unused1[170]; /* 0x26 */
 	char board_name[11]; /* 0xd0 */
 	char unused2[5]; /* 0xdc */