dmaengine: sun6i: Fix memcpy operation

The prep_memcpy call was not setting any meaningful burst and width because it
was relying on the dma_slave_config was not set already.

Rework the needed conversion functions, and hardcode the width and burst to
use.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: stable@vger.kernel.org
Signed-off-by: Vinod Koul <vinod.koul@intel.com>