[PATCH] Altix: more ACPI PRT support

The SN Altix platform does not conform to the IOSAPIC IRQ routing model.
Add code in acpi_unregister_gsi() to check if (acpi_irq_model ==
ACPI_IRQ_MODEL_PLATFORM) and return.

Due to an oversight, this code was not added previously when
similar code was added to acpi_register_gsi().

http://marc.theaimsgroup.com/?l=linux-acpi&m=116680983430121&w=2

Signed-off-by: John Keller <jpk@sgi.com>
Acked-by: Len Brown <lenb@kernel.org>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>