device/pci_device.c: Return if the scan parameter is invalid

Clang is unhappy about codepath of an invalid parameter because
variables remain unset.

Change-Id: I1ba392a48cf3f81a29d9645e5cf220b122d588af
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63038
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/device/pci_device.c

@@ -1518,6 +1518,8 @@ static void pci_bridge_route(struct bus *link, scan_state state)
 		primary = parent->secondary;
 		secondary = link->secondary;
 		subordinate = link->subordinate;
+	} else {
+		return;
 	}
 
 	if (state == PCI_ROUTE_SCAN) {