Revert "ACPICA: Permanently set _REV to the value '2'."

Revert commit b1ef2972

 (ACPICA: Permanently set _REV to the value
'2'.) as it causes a sound regression to happen on Dell XPS 13 (2015).

Reported-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>