RDMA/usnic: Stop using iommu_present()

Even if an IOMMU might be present for some PCI segment in the system,
that doesn't necessarily mean it provides translation for the device(s)
we care about. Replace iommu_present() with a more appropriate check at
probe time, and garbage-collect the resulting empty init function.

Link: https://lore.kernel.org/r/f707b4248e1d33b6d2c7f1d7c94febb802cf9890.1649161199.git.robin.murphy@arm.com


Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>