dmaengine: ioatdma: Fix variable array length

Sparse reported:
drivers/dma/ioat/prep.c:637:27: sparse: Variable length array is used.

Assigning a static value for the array.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>