ASoC: fsi: modify variable name to easy to understand

Current FSI driver is using
data-length / width / number / offset for variables.
But it was a very confusing name.

This patch rename them to easy to understand,
and add new functions for it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>