dmaengine: ioat: remove unused ‘ioat_dma’

In ioat_tx_submit_unlock(), variable ‘ioat_dma’ is initialized but never
used, which leads to warning with W=1

drivers/dma/ioat/dma.c: In function ‘ioat_alloc_ring_ent’:
drivers/dma/ioat/dma.c:341:25: warning: variable ‘ioat_dma’ set but not used [-Wunused-but-set-variable]
  struct ioatdma_device *ioat_dma;

So remove it.

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