spi: s3c64xx: Extract FIFO depth calculation to a dedicated macro

Simplify the code by extracting all cases of FIFO depth calculation into
a dedicated macro. No functional change.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://msgid.link/r/20240120170001.3356-1-semen.protsenko@linaro.org


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