dmaengine: vdma: Use dma_pool_zalloc

Dma_pool_zalloc combines dma_pool_alloc and memset 0.  The semantic patch
that makes this transformation is as follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
expression d,e;
statement S;
@@

        d =
-            dma_pool_alloc
+            dma_pool_zalloc
             (...);
        if (!d) S
-       memset(d, 0, sizeof(*d));
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>