x86, irq, ACPI: Change __acpi_register_gsi to return IRQ number instead of GSI

Currently __acpi_register_gsi is defined to return GSI number and
may be set to acpi_register_gsi_pic(), acpi_register_gsi_ioapic(),
acpi_register_gsi_xen_hvm() and acpi_register_gsi_xen().

Among which, acpi_register_gsi_ioapic() returns GSI number, but
acpi_register_gsi_xen_hvm() and acpi_register_gsi_xen() actually
returns IRQ number instead of GSI. And for acpi_register_gsi_pic(),
GSI number equals to IRQ number.

So change acpi_register_gsi_ioapic() to return IRQ number, it also
simplifies the code.

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: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Grant Likely <grant.likely@linaro.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: Len Brown <len.brown@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Link: http://lkml.kernel.org/r/1402380887-32512-1-git-send-email-jiang.liu@linux.intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>