ASoC: dwc: Reorder code in preparation for DT support

Move code that configures the DAI and DMA into a separate function.  This
reduces the size of the dw_i2s_probe function and will make it easier to
add support for device tree to the driver.

Signed-off-by: Andrew Jackson <Andrew.Jackson@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>