dw_dmac: don't exceed AHB master number in dwc_get_data_width

The driver assumes that hardware has two AHB masters which might not be always
true. In such cases we must not exceed number of the AHB masters present in the
hardware. In the proposed scheme in this patch, we would choose the master with
highest possible number whenever we exceed max AHB masters.

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>