monitor: Use the number of failed tach sensors at startup

When marking a fan nonfunctional due to its tach sensors failing
to be read from dbus, a check against the configured number of
sensors that should result in the fan being marked as nonfunctional
should be checked.

Tested:
    Run phosphor-fan-monitor --monitor in witherspoon qemu,
    when the number of failed tach sensor is larger than the
    configured _numSensorFailsForNonFunc then mark the
    associated fan as nonfunctional.

Change-Id: I6ff97b9aae4279d6ce402d3aecda087d45dfa318
Signed-off-by: Jolie Ku <jolie_ku@wistron.com>
diff --git a/monitor/tach_sensor.cpp b/monitor/tach_sensor.cpp
index 078b213..01baada 100644
--- a/monitor/tach_sensor.cpp
+++ b/monitor/tach_sensor.cpp
@@ -97,7 +97,7 @@
             log<level::ERR>(
                 fmt::format("Failed to retrieve tach sensor {}", _name)
                     .c_str());
-            // mark tach sensor as nonfunctional
+            // Mark tach sensor as nonfunctional
             setFunctional(false);
             throw InvalidSensorError();
         }