PCI / pci-label: treat PCI label with index 0 as valid label

Current pci-label driver detects ACPI label by checking label index
returned by ACPI _DSM method, and treats it as valid if label index
is positive. According to ACPI Firmware specification 3.1, zero is
also an valid label index. So change code to detect availability of
ACPI slot label by checking availaiblity of ACPI _DSM function for
PCI label.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>