ASoC: fsl_sai: derive TX FIFO watermark from FIFO depth

The DMA request schould be triggered as soon as the FIFO has space
for another burst. As different versions of the SAI block have
different FIFO sizes, the watrmark level needs to be derived from
version specific data.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Angus Ainslie <angus@akkea.ca>
Reviewed-by: Angus Ainslie <angus@akkea.ca>
Link: https://lore.kernel.org/r/20190717105635.18514-3-l.stach@pengutronix.de


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