ASoC: atmel-pcm-dma: move prepare for dma to dai prepare

as prepare callback for dma is acctually access ssc register
which better done in dai driver, so move it to dai prepare
callback function

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>