src/mainboard: Const'ify pci_devfn_t devices

Change-Id: I5bb1a819475383719dbda32d9b5fea63da1e6713
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40611
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/biostar/a68n_5200/bootblock.c

@@ -35,7 +35,7 @@ void bootblock_mainboard_early_init(void)
 	pm_io_write8(0x24, 1);
 
 	/* Set LPC decode enables. */
-	pci_devfn_t dev = PCI_DEV(0, 0x14, 3);
+	const pci_devfn_t dev = PCI_DEV(0, 0x14, 3);
 	pci_write_config32(dev, 0x44, 0xff03ffd5);
 
 	/* enable SIO LPC decode */