ASoC: fsi: re-define fsi_is_play() and fsi_stream_is_play()

This patch re-define fsi_is_play() and fsi_stream_is_play().
fsi_data_pop/push() function keeps direct value of "is_play" at this point,
but it will be removed soon.
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>