PCI: dwc: Rename 'dw_pcie::link_gen' to 'dw_pcie::max_link_speed'

PCI: dwc: Rename 'dw_pcie::link_gen' to 'dw_pcie::max_link_speed'

The 'link_gen' field is now holding the maximum supported link speed set
either by the controller driver or by DT through 'max-link-speed'
property.

However, the name 'link_gen' sounds like the negotiated link speed of
the PCIe link.

So rename it to 'max_link_speed' to make it clear that it holds the
maximum supported link speed of the controller.

Link: https://lore.kernel.org/linux-pci/20240911-pci-qcom-gen4-stability-v7-1-743f5c1fd027@linaro.org
Tested-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>