dmaengine: idma64: make better performance on pause / resume

Accordingly to the documentation the CH_DRAIN bit enforses single bursts when
channel is going to be suspended. This, in case when channel will be resumed,
makes data to flow in non-optimal mode until DMA returns to full burst mode.
The fix differentiates pause / resume cycle from pause / terminate and sets
CH_DRAIN bit accordingly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>