device/pnp: introduce and use PNP_SKIP_FUNCTION

-1 shouldn't be assigned to an unsigned variable, so use an otherwise
unused constant here. Since 7 is the highest virtual LDN number, using
0xffff as PNP_SKIP_FUNCTION marker has no unwanted side effects.

Change-Id: I5e31e7ef9dad5fedfd5552963c298336c533a5e9
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37741
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/superio/smsc/smscsuperio/superio.c

@@ -280,7 +280,10 @@ static void enable_dev(struct device *dev)
 	 */
 	for (j = 0; j < ARRAY_SIZE(pnp_dev_info); j++) {
 		fn = pnp_dev_info[j].function;
-		pnp_dev_info[j].function = logical_device_table[i].devs[fn];
+		if (logical_device_table[i].devs[fn] != -1)
+			pnp_dev_info[j].function = logical_device_table[i].devs[fn];
+		else
+			pnp_dev_info[j].function = PNP_SKIP_FUNCTION;
 	}
 
 	/* Enable the specified devices (if present on the chip). */