[IA64] SN: prevent IRQ retargetting in request_irq()

With the introduction of the generic affinity autoselector,
irq_select_affinity(), IRQs are now being retargetted,
using a default mask, via the request_irq() path.
This results in all IRQs targetted at CPU 0.

SN Altix assigns affinity in the SN PROM, and does not
expect that to be changed as part of request_irq().

Set the IRQ_AFFINITY_SET flag to prevent
request_irq() from resetting affinity.

Signed-off-by: John Keller <jpk@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>