PCI: loongson: Limit MRRS to 256

This is a partial revert of 8b3517f8 ("PCI: loongson: Prevent LS7A MRRS
increases") for MIPS-based Loongson.

Some MIPS Loongson systems don't support arbitrary Max_Read_Request_Size
(MRRS) settings.  8b3517f8 ("PCI: loongson: Prevent LS7A MRRS
increases") worked around that by (1) assuming that firmware configured
MRRS to the maximum supported value and (2) preventing the PCI core from
increasing MRRS.

Unfortunately, some firmware doesn't set that maximum MRRS correctly, which
results in devices not being initialized correctly.  One symptom, from the
Debian report below, is this:

  ata4.00: exception Emask 0x0 SAct 0x20000000 SErr 0x0 action 0x6 frozen
  ata4.00: failed command: WRITE FPDMA QUEUED
  ata4.00: cmd 61/20:e8:00:f0:e1/00:00:00:00:00/40 tag 29 ncq dma 16384 out
           res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
  ata4.00: status: { DRDY }
  ata4: hard resetting link

Limit MRRS to 256 because MIPS Loongson with higher MRRS support is
considered rare.

This must be done at device enablement stage because the MRRS setting may
get lost if PCI_COMMAND_MASTER on the parent bridge is cleared, and we are
only sure parent bridge is enabled at this point.

Fixes: 8b3517f8 ("PCI: loongson: Prevent LS7A MRRS increases")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217680
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1035587
Link: https://lore.kernel.org/r/20231201115028.84351-1-jiaxun.yang@flygoat.com


Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Cc: stable@vger.kernel.org