ASoC: rsnd: ssi: remove unnesessary period_pos

period_pos can always be calculated by byte_pos and
byte_per_period, there is no reason to maintain this
variable in rsnd_dai_stream.

This patch removes period_pos from rsnd_ssi and calculates
next_period_byte with consideration of actual byte_pos value.

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