RDMA/usnic: Use device_iommu_capable()

Use the new interface to check the capability for our device
specifically.

Link: https://lore.kernel.org/r/96ffe7050da0aa0ad6bce4705c3532f3ecaf32e3.1654688682.git.robin.murphy@arm.com


Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>