dma: HDA ignore repeated start/stop requests

The DMA API contract specifies that start/stop may be called multiple
times. Prior to adding power management this was perfectly fine as it was.
In adding power management, there are additional side effects that can
cause issues. Instead check the state of the channel prior to start/stop
and do nothing if already in the desired state.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>