dmaengine: sh: make shdma_prep_dma_cyclic static

kbuild test robot reports that shdma_prep_dma_cyclic should be static, since
symbol is not declared, quick check revails that is the case

>> drivers/dma/sh/shdma-base.c:660:32: sparse: symbol 'shdma_prep_dma_cyclic'
>> was not declared. Should it be static?

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>