Revert "ACPICA: FADT support cleanup"

Pavel Machek reports that commit 6ea8c546 (ACPICA: FADT support
cleanup) breaks thermal management on his Thinkpad X60 and T40p, so
revert it.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=187311
Fixes: 6ea8c546 (ACPICA: FADT support cleanup)
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpica/tbfadt.c

@@ -480,19 +480,17 @@ static void acpi_tb_convert_fadt(void)
 	u32 i;
 
 	/*
-	 * For ACPI 1.0 FADTs (revision 1), ensure that reserved fields which
+	 * For ACPI 1.0 FADTs (revision 1 or 2), ensure that reserved fields which
 	 * should be zero are indeed zero. This will workaround BIOSs that
 	 * inadvertently place values in these fields.
 	 *
 	 * The ACPI 1.0 reserved fields that will be zeroed are the bytes located
 	 * at offset 45, 55, 95, and the word located at offset 109, 110.
 	 *
-	 * Note: The FADT revision value is unreliable because of BIOS errors.
-	 * The table length is instead used as the final word on the version.
-	 *
-	 * Note: FADT revision 3 is the ACPI 2.0 version of the FADT.
+	 * Note: The FADT revision value is unreliable. Only the length can be
+	 * trusted.
 	 */
-	if (acpi_gbl_FADT.header.length <= ACPI_FADT_V3_SIZE) {
+	if (acpi_gbl_FADT.header.length <= ACPI_FADT_V2_SIZE) {
 		acpi_gbl_FADT.preferred_profile = 0;
 		acpi_gbl_FADT.pstate_control = 0;
 		acpi_gbl_FADT.cst_control = 0;