dw_dmac: introduce total_len field in struct dw_desc

By this new field we distinguish a total length of the chain and the individual
length of each descriptor in the chain.

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>