ASoC: rsnd: update pointer more accurate

Current rsnd driver updates pointer when DMA transfer was finished
in DMA transfer mode. But PulseAudio requests more accurate
pointer update when timer mode.
This patch consider about DMA transfer residue and update more
accurate pointer.

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