dma-mapping: remove an unnecessary NULL check

We already dereferenced "dev" when we called get_dma_ops() so this NULL
check is too late.  We're not supposed to pass NULL "dev" pointers to
dma_alloc_attrs().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>