dmaengine: dw: drop useless LIST_HEAD

Drop LIST_HEAD where the variable it declares is never used.

Commit ab703f81 ("dmaengine: dw: lazy allocation of dma
descriptors") removed the uses, but not the declaration.

The semantic patch that fixes this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@
identifier x;
@@
- LIST_HEAD(x);
  ... when != x
// </smpl>

Fixes: ab703f81 ("dmaengine: dw: lazy allocation of dma descriptors")
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Vinod Koul <vkoul@kernel.org>