PCI: dwc: Add dw_pcie_link_set_max_link_width()

This is a preparation before adding the Max-Link-width capability
setup which would in its turn complete the max-link-width setup
procedure defined by Synopsys in the HW-manual.

Seeing there is a max-link-speed setup method defined in the DW PCIe
core driver it would be good to have a similar function for the link
width setup.

That's why we need to define a dedicated function first from already
implemented but incomplete link-width setting up code.

Link: https://lore.kernel.org/linux-pci/20231018085631.1121289-3-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>