can: mcp251xfd: mcp251xfd_regmap_crc_read(): improve workaround handling for mcp2517fd

can: mcp251xfd: mcp251xfd_regmap_crc_read(): improve workaround handling for mcp2517fd

commit 406cc9cdb3e8d644b15e8028948f091b82abdbca upstream.

The mcp251xfd compatible chips have an erratum ([1], [2]), where the
received CRC doesn't match the calculated CRC. In commit
c7eb923c3caf ("can: mcp251xfd: mcp251xfd_regmap_crc_read(): work
around broken CRC on TBC register") the following workaround was
implementierend.

- If a CRC read error on the TBC register is detected and the first
  byte is 0x00 or 0x80, the most significant bit of the first byte is
  flipped and the CRC is calculated again.
- If the CRC now matches, the _original_ data is passed to the reader.
  For now we assume transferred data was OK.

Measurements on the mcp2517fd show that the workaround is applicable
not only of the lowest byte is 0x00 or 0x80, but also if 3 least
significant bits are set.

Update check on 1st data byte and workaround description accordingly.

[1] mcp2517fd: DS80000792C: "Incorrect CRC for certain READ_CRC commands"
[2] mcp2518fd: DS80000789C: "Incorrect CRC for certain READ_CRC commands"

Link: https://lore.kernel.org/all/DM4PR11MB53901D49578FE265B239E55AFB7C9@DM4PR11MB5390.namprd11.prod.outlook.com
Fixes: c7eb923c3caf ("can: mcp251xfd: mcp251xfd_regmap_crc_read(): work around broken CRC on TBC register")
Cc: stable@vger.kernel.org
Reported-by: Pavel Modilaynen <pavel.modilaynen@volvocars.com>
Signed-off-by: Thomas Kopp <thomas.kopp@microchip.com>
[mkl: split into 2 patches, update patch description and documentation]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>