VT-d: use proper error codes in iommu_enable_x2apic_IR()

VT-d: use proper error codes in iommu_enable_x2apic_IR()

... allowing to suppress a confusing message combination: When
ACPI_DMAR_X2APIC_OPT_OUT is set, so far we first logged a message
that IR could not be enabled (hence not using x2APIC), followed by
one indicating successful initialization of IR (if no other problems
prevented that).

Also adjust the return type of iommu_supports_eim() and fix some
broken indentation in the function.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Yang Zhang <yang.z.zhang@intel.com>