ASoC: rcar: clear DE bit only in PDMACHCR when it stops

R-Car datasheet indicates "Clear DE in PDMACHCR" for transfer stop,
but current code clears all bits in PDMACHCR.
Because of this, DE bit might never been cleared,
and it causes CMD overflow. This patch fixes this issue.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>