chromeec: Fix ACPI compile warning

For system without secondary battery, current DSDT will report warning
during build time. Add a conditional check to make sure only battery
index 0 can return success.

TEST=Build pass.

Change-Id: Iae12c5d1aa749948ef4025c8b5e60c97e1b747a5
Signed-off-by: Lijian Zhao <lijian.zhao@intel.com>
Reviewed-on: https://review.coreboot.org/23661
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/ec/google/chromeec/acpi/battery.asl

@@ -42,6 +42,10 @@ Method (BTSW, 1)
 			Return (One)
 		}
 	}
+#else
+	If (LNotEqual (0, Arg0)) {
+		Return (One)
+	}
 #endif
 	Return (Zero)
 }