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/include/device/pnp.h

@@ -34,6 +34,7 @@ extern struct device_operations pnp_ops;
 
 struct pnp_info {
 	struct device_operations *ops; /* LDN-specific ops override */
+#define PNP_SKIP_FUNCTION 0xffff
 	unsigned int function; /* Must be at least 16 bits (virtual LDNs)! */
 	unsigned int flags;
 #define PNP_IO0  0x000001