dw_dmac: fill individual length of descriptor

It will be useful to have the length of the transfer in the descriptor. The
cyclic transfer functions remained untouched.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>