ASoC: fsl_sai: Use better variable names

ASoC: fsl_sai: Use better variable names

"ret" is normally used as a variable name for return values. In
fsl_sai_set_bclk() it stores the difference between the desired rate and
the rate we can archieve, so rename it to "diff". Also rename "savesub"
to "bestdiff" as that stores the best difference we have found.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20220302083428.3804687-6-s.hauer@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>