PCI: Rework pcie_retrain_link() wait loop

Transform wait code to a "do {} while (time_before())" loop as recommended
by reviewer.  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>