iommu/dma: Remove redundant "!dev" checks

iommu_dma_init_domain() is now only called from iommu_setup_dma_ops(),
which has already assumed dev to be non-NULL.

Reviewed-by: John Garry <john.garry@huawei.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/06024523c080364390016550065e3cfe8031367e.1628682049.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>