Revert "PCI: tegra194: Enable support for 256 Byte payload"

After commit 4fb8e46c ("PCI: tegra194: Enable support for 256 Byte
payload"), we initialize MPS=256 for tegra194 Root Ports before enumerating
the hierarchy.

Consider an Endpoint that supports only MPS=128.  In the default situation
(CONFIG_PCIE_BUS_DEFAULT set and no "pci=pcie_bus_*" parameter), Linux
tries to configure the MPS of every device to match the upstream bridge.
If the Endpoint is directly below the Root Port, Linux can reduce the Root
Port MPS to 128 to match the Endpoint.  But if there's a switch in the
middle, Linux doesn't reduce the Root Port MPS because other devices below
the switch may already be configured with MPS larger than 128.

This scenario results in uncorrectable Malformed TLP errors if the Root
Port sends TLPs with payloads larger than 128 bytes.  These errors can
be avoided by using the "pci=pcie_bus_safe" parameter, but it doesn't
seem to be a good idea to always have this parameter even for basic
functionality to work.

Revert commit 4fb8e46c ("PCI: tegra194: Enable support for 256 Byte
payload") so the Root Ports default to MPS=128, which all devices
support.

If peer-to-peer DMA is not required, one can use "pci=pcie_bus_perf" to
get the benefit of larger MPS settings.

[bhelgaas: commit log; kwilczynski: retain "u16 val_16" declaration at
the top, add missing acked by tag]
Fixes: 4fb8e46c ("PCI: tegra194: Enable support for 256 Byte payload")
Link: https://lore.kernel.org/linux-pci/20230619102604.3735001-1-vidyas@nvidia.com


Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Cc: stable@vger.kernel.org # v6.0-rc1+