monitor: use signed value for factor and offset

The code was using size_t for factor and offset. For some fans the
offset could be negative, so it needs to use signed types.
This commit changes the offset type to int64_t.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I7443b00cd9f55ff0acb2fcc077bf2f5ba2b74395

monitor/tach_sensor.cpp

@@ -75,7 +75,7 @@ TachSensor::TachSensor(Mode mode,
                        size_t funcDelay,
                        const std::string& interface,
                        size_t factor,
-                       size_t offset,
+                       int64_t offset,
                        size_t timeout,
                        const sdeventplus::Event& event) :
     _bus(bus),

monitor/tach_sensor.hpp

@@ -89,7 +89,7 @@ class TachSensor
                    size_t funcDelay,
                    const std::string& interface,
                    size_t factor,
-                   size_t offset,
+                   int64_t offset,
                    size_t timeout,
                    const sdeventplus::Event& event);
 
@@ -133,7 +133,7 @@ class TachSensor
         /**
          * @brief Returns the offset of the sensor target
          */
-        inline size_t getOffset() const
+        inline int64_t getOffset() const
         {
             return _offset;
         }
@@ -277,7 +277,7 @@ class TachSensor
         /**
          * @brief The offset of target to get fan rpm
          */
-        const size_t _offset;
+        const int64_t _offset;
 
         /**
          * @brief The input speed, from the Value dbus property

monitor/types.hpp

@@ -44,7 +44,7 @@ using SensorDefinition = std::tuple<std::string,
                                     bool,
                                     std::string,
                                     size_t,
-                                    size_t>;
+                                    int64_t>;
 
 constexpr auto fanNameField = 0;
 constexpr auto funcDelay = 1;