dmaengine: IOATDMA: fix timer code that continues to restart channels during idle

The timer_event() function seems to have a bug where it ends up marking the
last entry as non-responding and eventually attempts to restart the
channel.  This also continuously happen when idle. What needs to happen is
for us to make sure there are no descriptors active and then handle that
case properly.  We should only hit the "cleanup" stage if there are still
active descriptors.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>