ASoC: fsi: data push/pop calculation part was divided

Next transfer data size of "push" and "pop" had calculated on shared function.
But it was not readable code.
This patch divided it into for push, and for pop.

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