dma-mapping: fix inverted logic in dma_supported

The cleanup in commit 356da6d0 ("dma-mapping: bypass indirect calls
for dma-direct") accidentally inverted the logic in the check for the
presence of a ->dma_supported() callback. Switch this back to the way it
was to prevent a crash on boot.

Fixes: 356da6d0 ("dma-mapping: bypass indirect calls for dma-direct")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>