ASoC: ux500_pcm: Differentiate between pdata and DT initialisation

If booting with full DT support (i.e. DMA too, the last piece of the
puzzle), then we don't need to use the compatible_request_channel call
back or require some of the historical bumph which probably isn't
required by a platform data start-up now either. This will also be
ripped out in upcoming commits.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>