dmaengine: cppi41: Fix an Oops happening in cppi41_dma_probe()

This fix an Oops happening on all platforms using the old dt bindings
(all platforms but da8xx).
This update cppi41_dma_probe() to use the index variable which is
required to keep compatibility between old and new dt bindings.

Fixes: 8e3ba95f ("dmaengine: cppi41: use managed functions devm_*()")
Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>