spi: spi-fsl-lpspi: Treat prescale_max == 0 as no erratum

spi: spi-fsl-lpspi: Treat prescale_max == 0 as no erratum

This erratum only ever results in a max value of 1, otherwise the full 3
bits are available. To avoid repeating the same default prescale value
for every new device's devdata, treat 0 as no limit (7) and only set a
value when the erratum is present.

Change the field to be 3 bits to catch out of range definitions.

No functionality change.

Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250828-james-nxp-lpspi-v2-7-6262b9aa9be4@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>