dmaengine: ioat: statify symbol

Sparse warns:
drivers/dma/ioat/init.c:1215:6: warning: symbol 'ioat_resume' was not declared. Should it be static?

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