IOMMU/x86: tidy adjust_irq_affinities hook

IOMMU/x86: tidy adjust_irq_affinities hook

As of 3e56754b0887 ("xen/cet: Fix __initconst_cf_clobber") there's no
need for a non-void return value anymore, as the hook functions are no
longer themselves passed to __initcall(). For the same reason the
iommu_enabled checks can now move from the individual functions to the
wrapper.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>