ACPI: check acpi_disabled in acpi_table_parse() and acpi_table_parse_entries()

Allow consumers of the acpi_table_parse()/acpi_table_parse_entries() API
to gracefully handle the acpi_disabled=1 case via return value
rather than checking the global flag themselves.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>