[IA64] SN: Correctly update smp_affinty mask

On Altix systems, the /proc/irq/nn/smp_affinity mask is not being setup
at device iniitalization, or updated after an interrupt redirection.
This patch resolves those issues.

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