PCI: Add pcie_get_width_cap() to find max supported link width

Add pcie_get_width_cap() to find the max link width supported by a device.
Change max_link_width_show() to use pcie_get_width_cap().

Signed-off-by: Tal Gilboa <talgi@mellanox.com>
[bhelgaas: return width directly instead of error and *width, don't export
outside drivers/pci]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>