dmaengine: stm32-dmamux: Switch to use device_property_count_u32()

Use use device_property_count_u32() directly, that makes code neater.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190723190757.67351-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>