Blackfin arch: flush/inv the correct range when using write back cache and fix bugs find by dmacopy

 - flush/inv the correct range
 - dmacopy test failed when policy is write_back - invalidate before dma
   http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3367


   It's the cache invalidate what is causing the issue.
   There is no invalidate only instruction it's always: FLUSHINV
   So when we "invalidate" after the DMA we might (do) overwrite freshly 
   dma'ed data by dirty Cache WB content.

   Fixed by moving the "invalidate" at the beginning of dma_memcpy.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>