PCI: Factor out pcie_retrain_link() function

commit 86fa6a344209d9414ea962b1f1ac6ade9dd7563a upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>