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>