dw_dmac: apply default dma_mask if needed

In some cases we got the device without dma_mask configured. We have to apply
the default value to avoid crashes during memory mapping.

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>