sfc: Improve test for IOMMU in use

The device::iommu_group field may be set even if no IOMMU is in use.
iommu_present() is still a better indicator, although it doesn't tell
us whether *our* device is affected.

Reported-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>