ibmvnic: Fix DMA mapping mistakes

Fix some mistakes caught by the DMA debugger. The first change
fixes a unnecessary unmap that should have been removed in an
earlier update. The next hunk fixes another bad unmap by zeroing
the bit checked to determine that an unmap is needed. The final
change fixes some buffers that are unmapped with the wrong
direction specified.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>