sb/intel/i82371eb/isa: make IOAPIC ID const

Since the local IOAPIC ID variable is initialized as 2 and never changed
afterwards, so make it const to make this more obvious.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I1f19cc43b44a938758a43346f4fa75f8ed39ddea
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80349
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/southbridge/intel/i82371eb/isa.c

@@ -53,7 +53,7 @@ static void isa_init(struct device *dev)
 	 */
 	if (CONFIG(IOAPIC)) {
 		u16 reg16;
-		u8 ioapic_id = 2;
+		const u8 ioapic_id = 2;
 
 		/* Enable IOAPIC. */
 		reg16 = pci_read_config16(dev, XBCS);