spi: fsi: Reduce max transfer size to 8 bytes

Security changes have forced the SPI controllers to be limited to
8 byte reads. Refactor the sequencing to just handle 8 bytes at a
time.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20210716133915.14697-2-eajames@linux.ibm.com


Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org