acpi_register_gsi() should return a negative value in case of failure.

Currently, it returns the return value from irq_create_fwspec_mapping().
However, irq_create_fwspec_mapping() returns 0 for failure. Fix the
issue by returning -EINVAL if irq_create_fwspec_mapping() returns zero.

Fixes: d44fa3d4 ("ACPI: Add support for ResourceSource/IRQ domain mapping")
Cc: 4.11+ <stable@vger.kernel.org> # 4.11+
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
[ rjw: Rename a new local variable ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>