[IA64] sn_check_intr: use ia64_get_irr()

Use the recently-added ia64_get_irr() rather than duplicating the code.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: Jes Sorensen <jes@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>