openrisc: fix cache maintainance the the sync_single_for_device DMA operation

The cache maintaince in the sync_single_for_device operation should be
equivalent to the map_page operation to facilitate reusing buffers.  Fix the
openrisc implementation by moving the cache maintaince performed in map_page
into the sync_single method, and calling that from map_page.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Stafford Horne <shorne@gmail.com>