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/device/pnp_device.c

@@ -370,7 +370,7 @@ void pnp_enable_devices(struct device *base_dev, struct device_operations *ops,
 	/* Setup the ops and resources on the newly allocated devices. */
 	for (i = 0; i < functions; i++) {
 		/* Skip logical devices this Super I/O doesn't have. */
-		if (info[i].function == -1)
+		if (info[i].function == PNP_SKIP_FUNCTION)
 			continue;
 
 		path.pnp.device = info[i].function;