[IA64] irq_ia64.c typo s/_IA64_REG_AR_SP/_IA64_REG_SP/

Patch supplied by Sourav Sen from HP:
  If I turn on IRQ_DEBUG in arch/ia64/kernel/irq_ia64.c in 2.6.8.1,
  the compilation does not go thru as there isn't any _IA64_REG_AR_SP
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/irq_ia64.c

@@ -115,7 +115,7 @@ ia64_handle_irq (ia64_vector vector, struct pt_regs *regs)
 		 * switched atomically.
 		 */
 		bsp = ia64_getreg(_IA64_REG_AR_BSP);
-		sp = ia64_getreg(_IA64_REG_AR_SP);
+		sp = ia64_getreg(_IA64_REG_SP);
 
 		if ((sp - bsp) < 1024) {
 			static unsigned char count;