[AVR32] Fix incorrect invalidation of shared cachelines

Fix bug in dma_map_single(..., DMA_FROM_DEVICE) caused by incorrect
invalidation of shared cachelines at the beginning and/or end of
the specified buffer.  Those shared cachelines need to be flushed,
since they may hold valid data (which must not be discarded).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>