dw_dmac: backlink to dw_dma in dw_dma_chan is superfluous

The same information could be extracted from the struct dma_chan.
The patch introduces helper function dwc_get_data_width() as well.

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>