ASoC: rsnd: tidyup PIO/DMA mode settings method

Current ssi.c has .cr_etc which is used for SSICR's
etc settings. but, it is used as PIO/DMA switching purpose now.
This patch tidyup this method. This is prepare for
under/over run issue handling

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