dmaengine: remove struct scatterlist for header

Commit 90b44f8f introduces dmaengine_prep_slave_single API which adds
scatterlist.h in dmaengine.h, so defining struct scatterlist is not required

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>