arch/x86/acpi/globutil.asl: Make control method `S2BF` serialized

This changes eliminates a warning from the ASL compiler.

Change-Id: I502cca731b6b4cd3e17c57fc191f1eed10a5a1fe
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/4093
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)

src/arch/x86/acpi/globutil.asl

@@ -39,7 +39,7 @@ Method(SLEN, 1)
 	Return(Sizeof(Local0))
 }
 
-Method(S2BF, 1)
+Method(S2BF, 1, Serialized)
 {
 	Add(SLEN(Arg0), One, Local0)
 	Name(BUFF, Buffer(Local0) {})