PCI: rockchip-host: Do not attempt 5.0 GT/s retraining

Drop the 5.0 GT/s Link Speed retraining from Rockchip PCIe Root Complex
Mode Operation, so-called host driver.

The reason is that Shawn Lin from Rockchip has reiterated that there may be
danger of "catastrophic failure" in using their PCIe with 5.0GT/s speeds.

While Rockchip has done so informally without issuing a proper errata, and
the particulars are thus unknown, this may cause data loss or worse.

This change is corroborated by RK3399 official datasheet [1], which states
maximum link speed for this platform is 2.5 GT/s.

[1] https://opensource.rock-chips.com/images/d/d7/Rockchip_RK3399_Datasheet_V2.1-20200323.pdf

Fixes: e77f847df54c ("PCI: rockchip: Add Rockchip PCIe controller support")
Reported-by: Dragan Simic <dsimic@manjaro.org>
Reported-by: Shawn Lin <shawn.lin@rock-chips.com>
Link: https://lore.kernel.org/all/ffd05070-9879-4468-94e3-b88968b4c21b@rock-chips.com/
Signed-off-by: Geraldo Nascimento <geraldogabriel@gmail.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/68f6353e3a2ea4914de36c42b6906e41282adad3.1781622998.git.geraldogabriel@gmail.com

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