ACPI, irq, x86: Return IRQ instead of GSI in mp_register_gsi()

Function mp_register_gsi() returns blindly the GSI number for the ACPI
SCI interrupt. That causes a regression when the GSI for ACPI SCI is
shared with other devices.

The regression was caused by commit 84245af7297ced9e8fe "x86, irq, ACPI:
Change __acpi_register_gsi to return IRQ number instead of GSI" and
exposed on a SuperMicro system, which shares one GSI between ACPI SCI
and PCI device, with following failure:

http://sourceforge.net/p/linux1394/mailman/linux1394-user/?viewmonth=201410
[    0.000000] ACPI: INT_SRC_OVR (bus 0 bus_irq 9 global_irq 20 low
level)
[    2.699224] firewire_ohci 0000:06:00.0: failed to allocate interrupt
20

Return mp_map_gsi_to_irq(gsi, 0) instead of the GSI number.

Reported-and-Tested-by: Daniel Robbins <drobbins@funtoo.org>
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Len Brown <len.brown@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: <stable@vger.kernel.org> # 3.17
Link: http://lkml.kernel.org/r/1414387308-27148-4-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/acpi/boot.c

@@ -397,7 +397,7 @@ static int mp_register_gsi(struct device *dev, u32 gsi, int trigger,
 
 	/* Don't set up the ACPI SCI because it's already set up */
 	if (acpi_gbl_FADT.sci_interrupt == gsi)
-		return gsi;
+		return mp_map_gsi_to_irq(gsi, IOAPIC_MAP_ALLOC);
 
 	trigger = trigger == ACPI_EDGE_SENSITIVE ? 0 : 1;
 	polarity = polarity == ACPI_ACTIVE_HIGH ? 0 : 1;