dmaengine: Document dmaengine_prep_dma_memset

Document this function to make clear the expected behavior of the
'value' parameter. It was intended to match the behavior of POSIX memset
as laid out here:

https://lore.kernel.org/dmaengine/YejrA5ZWZ3lTRO%2F1@matsya/



Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Link: https://lore.kernel.org/r/20220301182551.883474-2-benjamin.walker@intel.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>