ASoC: wm8997: Use async writes

When writing the patch write to the device asynchronously, allowing better
performance when used with a bus like SPI which supports this by
minimising the need to context switch back to the driver to get the
next bit of data.

Signed-off-by: Mark Brown <broonie@linaro.org>
Reviewed-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>