ACPI: Fix IASL Warning about unused method for _OSI check

According to the ACPI Spec for CondRefOf, the result argument is
optional.  In all of these locations, it was getting set but not
used, creating a warning in new versions of IASL.  Since it's
an optional argument, just remove it.

dsdt.aml     22:   if(CondRefOf(\_OSI,Local1))
Warning  3144 -                           ^
Method Local is set but never used (Local1)

Change-Id: I07f49ac5a3708838d1c4a7216dfb11acc415c881
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12692
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/mainboard/packardbell/ms2290/acpi/platform.asl

@@ -96,7 +96,7 @@ Scope(\_SB)
 		/* Let's assume we're running at least Windows 2000 */
 		Store (2000, OSYS)
 
-		If (CondRefOf(_OSI, Local0)) {
+		If (CondRefOf(_OSI)) {
 			If (_OSI("Windows 2001")) {
 				Store (2001, OSYS)
 			}