PCI: of: Zero max-link-speed value is invalid

Interpret zero value of max-link-speed property as invalid,
as the device tree bindings documentation specifies.

Link: https://lore.kernel.org/r/20200430080625.26070-4-pali@kernel.org
Tested-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

drivers/pci/of.c

@@ -592,7 +592,7 @@ int of_pci_get_max_link_speed(struct device_node *node)
 	u32 max_link_speed;
 
 	if (of_property_read_u32(node, "max-link-speed", &max_link_speed) ||
-	    max_link_speed > 4)
+	    max_link_speed == 0 || max_link_speed > 4)
 		return -EINVAL;
 
 	return max_link_speed;