[POWERPC] iSeries: fix irq.c for combined build

Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/irq.c

@@ -268,7 +268,8 @@ void do_IRQ(struct pt_regs *regs)
 	set_irq_regs(old_regs);
 
 #ifdef CONFIG_PPC_ISERIES
-	if (get_lppaca()->int_dword.fields.decr_int) {
+	if (firmware_has_feature(FW_FEATURE_ISERIES) &&
+			get_lppaca()->int_dword.fields.decr_int) {
 		get_lppaca()->int_dword.fields.decr_int = 0;
 		/* Signal a fake decrementer interrupt */
 		timer_interrupt(regs);