dmaengine: xdmac: Rework the chaining logic

So far, we were setting the NDE bit in our descriptors through some logic to
try to see if we were the last descriptor in the chain.

However, that was turning out to be rather complex to get right, while this
information is also available when we actually chain a new descriptor after an
already existing one.

Simplify this by never setting NDE unless when we actually chain a descriptor.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>