[IA64] Clean up CPE handler registration

Use local_vector_to_irq() instead of looping through all NR_IRQS.
This avoids registering the CPE handler on multiple irqs.  Only
register if the irq is valid.  If no valid irq is found, print an
error message and set up polling.

Signed-off-by: Russ Anderson <rja@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>