x86/irq: Fix regression caused by commit b568b8601f05

Commit b568b8601f05 ("Treat SCI interrupt as normal GSI interrupt")
accidently removes support of legacy PIC interrupt when fixing a
regression for Xen, which causes a nasty regression on HP/Compaq
nc6000 where we fail to register the ACPI interrupt, and thus
lose eg. thermal notifications leading a potentially overheated
machine.

So reintroduce support of legacy PIC based ACPI SCI interrupt.

Reported-by: Ville Syrjälä <syrjala@sci.fi>
Tested-by: Ville Syrjälä <syrjala@sci.fi>
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: <stable@vger.kernel.org> # 3.19+
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Cc: linux-pm@vger.kernel.org
Link: http://lkml.kernel.org/r/1424052673-22974-1-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/acpi/boot.c

@@ -613,6 +613,11 @@ int acpi_gsi_to_irq(u32 gsi, unsigned int *irqp)
 {
 	int rc, irq, trigger, polarity;
 
+	if (acpi_irq_model == ACPI_IRQ_MODEL_PIC) {
+		*irqp = gsi;
+		return 0;
+	}
+
 	rc = acpi_get_override_irq(gsi, &trigger, &polarity);
 	if (rc == 0) {
 		trigger = trigger ? ACPI_LEVEL_SENSITIVE : ACPI_EDGE_SENSITIVE;