PCI: Factor out pcie_retrain_link() function

Factor out pcie_retrain_link() to use for Pericom Retrain Link quirk.  No
functional change intended.

Signed-off-by: Stefan Mätje <stefan.maetje@esd.eu>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
CC: stable@vger.kernel.org