dw_dmac: introduce to_dw_desc() macro

The to_dw_desc() macro helps to retrieve the dw_desc node from the
corresponding list_head structure.

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

diff --cc drivers/dma/dw_dmac.c
Simple merge

diff --cc drivers/dma/dw_dmac_regs.h
Simple merge