ASoC: rsnd: dma.c: spin lock is no longer needed in IRQ handler

DMA handler had needed to calculate pointer before, but it doesn't
need now. Thus, we can remove unnecessary spin lock from DMAC handler.

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