ASoC: stm32: i2s: fix 32 bits channel length without mclk

Fix divider calculation in the case of 32 bits channel
configuration, when no master clock is used.

Fixes: e4e6ec7b ("ASoC: stm32: Add I2S driver")

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://lore.kernel.org/r/20211117104404.3832-1-olivier.moysan@foss.st.com


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