TachSensor: fix checkThresholds regression

Due to a merge mistake, the following commit
introduced an unintended change.

https://github.com/openbmc/dbus-sensors/commit/da98f095f25cc75af3c0268643bde2b21f5503e0#diff-33f4e0882e5eaea910f455ab4f5f22b2b9274ff46dd6120b6b23a092607174a0L187

Revert the code back.

Signed-off-by: Zhikui Ren <zhikui.ren@intel.com>
Change-Id: I17198e9cfece47022a437cdba12eea8c3e4b32e2
diff --git a/src/TachSensor.cpp b/src/TachSensor.cpp
index c375dbf..2c48cc9 100644
--- a/src/TachSensor.cpp
+++ b/src/TachSensor.cpp
@@ -183,8 +183,7 @@
 
 void TachSensor::checkThresholds(void)
 {
-    // WA - treat value <= 0 as not present
-    bool status = false;
+    bool status = thresholds::checkThresholds(this);
 
     if (redundancy && *redundancy)
     {