dmaengine: hsu: speed up residue calculation

There is no need to calculate an overall length of the descriptor each time we
call for DMA transfer status. Instead we do this at descriptor allocation stage
and keep the stored length for further usage.

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