dmaengine: pl330: fix residual for non-running BUSY descriptors

Only one descriptor in the work list should be running at
any given time, but it's possible to have an enqueued BUSY
descriptor that has not yet transferred any data, or for
a BUSY descriptor to linger briefly before transitioning
to DONE. These cases should be handled to keep residual
calculations consistent even with the non-running BUSY
descriptors in the work list.

Signed-off-by: Stephen Barber <smbarber@chromium.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>