spi: s3c64xx: determine the fifo depth only once

Determine the FIFO depth only once, at probe time.
``sdd->fifo_depth`` can be set later on with the FIFO depth
specified in the device tree.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://msgid.link/r/20240216070555.2483977-5-tudor.ambarus@linaro.org


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