ASoC: dwc: Prepare clock before use

Some I2S clocks may require some time to get the clock ready
for operation and so need to be prepared before they are enabled.
So, prepare the clock as well as enabling it, but combine the
two through clk_prepare_enable.

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