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>
diff --git a/src/mainboard/lippert/frontrunner-af/dsdt.asl b/src/mainboard/lippert/frontrunner-af/dsdt.asl
index 86eccb2..eaac895 100644
--- a/src/mainboard/lippert/frontrunner-af/dsdt.asl
+++ b/src/mainboard/lippert/frontrunner-af/dsdt.asl
@@ -404,7 +404,7 @@
 
 			if(LNotEqual(OSVR, Ones)) {Return(OSVR)}	/* OS version was already detected */
 
-			if(CondRefOf(\_OSI,Local1))
+			if(CondRefOf(\_OSI))
 			{
 				Store(1, OSVR)                /* Assume some form of XP */
 				if (\_OSI("Windows 2006"))      /* Vista */