ASoC: fsl_sai: simplify register poking in fsl_sai_set_bclk

ASoC: fsl_sai: simplify register poking in fsl_sai_set_bclk

Depending on SAI synchronization mode, the same value is either
written to FSL_SAI_TCR2 or FSL_SAI_RCR2 or nothing is written at all.

As the computation is the same either way, factor it out to make it
clearer what the difference is. No functional change.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20220302083428.3804687-4-s.hauer@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>