sh: use dma_direct_ops for the CONFIG_DMA_COHERENT case

This is a slight change in behavior as we avoid the detour through the
virtual mapping for the coherent allocator, but if this CPU really is
coherent that should be the right thing to do.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Yoshinori Sato <ysato@users.sourceforge.jp>