ASoC: SOF: PM: add check before setting d0_substate

Add check before seeting d0_substate and return success if Audio DSP is
already in the target substate.

Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191111223343.19986-3-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>