PCI: imx6: Remove broken Gen2 workaround

Remove the remnants of the workaround for erratum ERR005184 which was never
completely implemented.  The checks alone don't carry any value as we don't
act properly on the result.

A workaround should be added to the lane speed change in establish_link
later.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>