ACPICA: Tables: Avoid SSDT installation with acpi_gbl_disable_ssdt_table_load.

It is reported that when acpi_gbl_disable_ssdt_table_load is specified, user
still can see it installed into /sys/firmware/acpi/tables on Linux boxes.
This is because the option only stops table "loading", but doesn't stop
table "installing", thus it is still in the acpi_gbl_root_table_list. With
previous cleanups, it is possible to prevent SSDT installations to make
it not such confusing.  The global variable is also renamed.  Lv Zheng.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
[rjw: Subject]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>