iommu/ipmmu-vmsa: Use helper functions to access dev->iommu_fwspec

Use the new helpers dev_iommu_fwspec_get()/set() to access
the dev->iommu_fwspec pointer. This makes it easier to move
that pointer later into another struct.

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