parisc: always use flush_kernel_dcache_range for DMA cache maintainance

Current the S/G list based DMA ops use flush_kernel_vmap_range which
contains a few UP optimizations, while the rest of the DMA operations
uses flush_kernel_dcache_range.  The single vs sg operations are supposed
to have the same effect, so they should use the same routines.  Use
the more conservation version for now, but if people more familiar with
parisc think the vmap version is generally fine for DMA we should switch
all interfaces over to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Helge Deller <deller@gmx.de>