ASoC: fsl_sai: use local device pointer

Use a local variable to dereference the device pointer once and use the
local variable in further calls. No functional changes.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Acked-by: Shengjiu Wang <shengjiu.wang@gmail.com>
Link: https://lore.kernel.org/r/20220601092342.3328644-1-m.felsch@pengutronix.de


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