spi: fsl_spi: Don't change speed while chipselect is active

Commit c9bfcb31 ("spi_mpc83xx: much improved driver") made
modifications to the driver to not perform speed changes while
chipselect is active. But those changes where lost with the
convertion to tranfer_one.

Previous implementation was allowing speed changes during
message transfer when cs_change flag was set.
At the time being, core SPI does not provide any feature to change
speed while chipselect is off, so do not allow any speed change during
message transfer, and perform the transfer setup in prepare_message
in order to set correct speed while chipselect is still off.

Reported-by: Herve Codina <herve.codina@bootlin.com>
Fixes: 64ca1a03 ("spi: fsl_spi: Convert to transfer_one")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/8aab84c51aa330cf91f4b43782a1c483e150a4e3.1671025244.git.christophe.leroy@csgroup.eu


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