media: xilinx: dma: Use media_pipeline_for_each_pad()

Replace usage of the deprecated media graph walk API with the new
media_pipeline_for_each_pad() macro.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>