PCI: Add function to obtain minimum link width and speed

A PCI Express device can potentially report a link width and speed which it will
not properly fulfill due to being plugged into a slower link higher in the
chain. This function walks up the PCI bus chain and calculates the minimum link
width and speed of this entire chain. This can be useful to enable a device to
determine if it has enough bandwidth for optimum functionality.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>