ASoC: fsi: use fsi_stream in fsi_get_current_fifo_samples() parameter

fsi_get_current_fifo_samples() uses fsi_stream instead of is_play.
This is just prepare cleanup for DMAEngine support.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>