dmaengine: sun6i: Restructure code to allow extension for new SoCs

dmaengine: sun6i: Restructure code to allow extension for new SoCs

The current code mixes three distinct operations when transforming
the slave config to register settings:

  1. special handling of DMA_SLAVE_BUSWIDTH_UNDEFINED, maxburst == 0
  2. range checking
  3. conversion of raw to register values

As the range checks depend on the specific SoC, move these out of the
conversion to distinct operations.

Signed-off-by: Stefan Brüns <[email protected]>
Acked-by: Maxime Ripard <[email protected]>
Signed-off-by: Vinod Koul <[email protected]>