spi: fsl-dspi: Preserve error code returned by dmaengine_slave_config()

dmaengine_slave_config() may return different error codes based on
the circumstances. Preserve it instead of shadowing to -EINVAL.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20240204203127.1186621-2-andy.shevchenko@gmail.com


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