spi: s3c64xx: avoid possible negative array index

The platform id is used as an index into the fifo_lvl_mask array.
Platforms can come with a negative device ID, PLATFORM_DEVID_NONE (-1),
thus we risked a negative array index. Catch such cases and fail to
probe.

Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20240207120431.2766269-6-tudor.ambarus@linaro.org


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