ibmveth: Fix DMA unmap error

Commit 6e8ab30e (ibmveth: Add scatter-gather support) introduced a
DMA mapping API inconsistency resulting in dma_unmap_page getting
called on memory mapped via dma_map_single. This was seen when
CONFIG_DMA_API_DEBUG was enabled. Fix up this API usage inconsistency.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Acked-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org> # v2.6.37+
Signed-off-by: David S. Miller <davem@davemloft.net>