Some ia64 machines don't use an iosapic interrupt controller (a
flagrant violation of the DIG64 spec), so don't assume it's there in
mca.c.  Platforms that provide a ACPI_INTERRUPT_CPEI vector are
responsible for registering its interrupt controller type in platform
specific code, iosapic.c:iosapic_register_platform_intr() does this
for platforms with an iosapic where the CPEI vector is listed in the
ACPI namespace, and on sn2, all external interrupts are assigned the
irq_type_sn type, so this change of assumption should be safe.