PCI: controller: Validate max-link-speed

PCI: controller: Validate max-link-speed

Add validation for the "max-link-speed" DT property in three more
drivers, using the pcie_get_link_speed() helper.

- brcmstb: If the value is missing or invalid, fall back to no
  limitation (pcie->gen = 0).  Fix the previous incorrect logic.
- mediatek-gen3: If the value is missing or invalid, use the maximum
  speed supported by the controller.
- rzg3s-host: If the value is missing or invalid, fall back to Gen2.

This ensures that all users of of_pci_get_max_link_speed() are ready
for the removal of the central range check.

Signed-off-by: Hans Zhang <18255117159@163.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20260313165522.123518-5-18255117159@163.com