ASoC: rsnd: PIO related function cleanup

SSI had shared counting pointer position method between PIO/DMA mode
before. But now DMA mode is using DMAEngine feature to get it.
Thus, this counting pointer position method is needed for only PIO mode.
We don't need to share code anymore.
This patch names PIO related functions as rsnd_ssi_pio_xxx(), and
merged/cleanuped each feature.

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