ASoC: wm_adsp: Disable DMAs before clearing the transfer length

This patch reorders the clearing of the DMA masks to avoid potential
artefacts being introduced.

Signed-off-by: Simon Trimmer <simont@opensource.wolfsonmicro.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>