dmaengine: dw: platform: check nr_masters to be non-zero

The value of nr_masters equal to 0 is invalid since this DMA controller has to
have at least one master.

Check this before we proceed with the rest of properties.

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