[mac] use Mac::ResetCounters() (#4315)

src/core/mac/mac.cpp

@@ -121,7 +121,7 @@ Mac::Mac(Instance &aInstance)
     randomExtAddress.GenerateRandom();
 
     mCcaSuccessRateTracker.Reset();
-    memset(&mCounters, 0, sizeof(otMacCounters));
+    ResetCounters();
     mExtendedPanId.Clear();
 
     mSubMac.Enable();
@@ -1814,11 +1814,6 @@ void Mac::SetPromiscuous(bool aPromiscuous)
     UpdateIdleMode();
 }
 
-void Mac::ResetCounters(void)
-{
-    memset(&mCounters, 0, sizeof(mCounters));
-}
-
 int8_t Mac::GetNoiseFloor(void)
 {
     return Get<Radio>().GetReceiveSensitivity();

src/core/mac/mac.hpp

@@ -567,7 +567,7 @@ public:
      * This method resets mac counters
      *
      */
-    void ResetCounters(void);
+    void ResetCounters(void) { memset(&mCounters, 0, sizeof(mCounters)); }
 
     /**
      * This method returns the MAC counter.