mb/pcengines/apu1/mainboard.c: Use 'PCI_BASE_ADDRESS_2' instead of magic number

Change-Id: Ibc2446d7b8d4334e26ca6335179f50b7abe301cb
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44831
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/pcengines/apu1/mainboard.c

@@ -301,7 +301,7 @@ const char *smbios_mainboard_serial_number(void)
 		return serial;
 
 	/* Read in the last 3 bytes of NIC's MAC address. */
-	bar18 = pci_read_config32(dev, 0x18);
+	bar18 = pci_read_config32(dev, PCI_BASE_ADDRESS_2);
 	bar18 &= 0xFFFFFFF0;
 	for (i = 3; i < 6; i++) {
 		mac_addr <<= 8;