[mac-types] add Clear() and Fill() method to Mac::ExtAddress (#4315)

The `Clear()` method clears the address (set all bytes to zero),
where as the `Fill()` method fills all bytes with a given value.

src/core/mac/mac_filter.cpp

@@ -234,7 +234,7 @@ otError Filter::GetNextRssIn(Iterator &aIterator, Entry &aEntry)
     // Return the default RssIn at the end of list
     if ((aIterator == OT_ARRAY_LENGTH(mFilterEntries)) && (mDefaultRssIn != kFixedRssDisabled))
     {
-        memset(&aEntry.mExtAddress, 0xff, OT_EXT_ADDRESS_SIZE);
+        static_cast<ExtAddress &>(aEntry.mExtAddress).Fill(0xff);
         aEntry.mRssIn = mDefaultRssIn;
         error         = OT_ERROR_NONE;
         aIterator++;

src/core/mac/mac_types.hpp

@@ -110,6 +110,20 @@ public:
         kReverseByteOrder, // Copy address bytes in reverse byte order.
     };
 
+    /**
+     * This method clears the Extended Address (sets all bytes to zero).
+     *
+     */
+    void Clear(void) { Fill(0); }
+
+    /**
+     * This method fills all bytes of address with a given byte value.
+     *
+     * @param[in] aByte A byte value to fill address with.
+     *
+     */
+    void Fill(uint8_t aByte) { memset(this, aByte, sizeof(*this)); }
+
     /**
      * This method generates a random IEEE 802.15.4 Extended Address.
      *

src/core/mac/sub_mac.cpp

@@ -61,7 +61,7 @@ SubMac::SubMac(Instance &aInstance)
     , mPcapCallbackContext(NULL)
     , mTimer(aInstance, &SubMac::HandleTimer, this)
 {
-    memset(mExtAddress.m8, 0, sizeof(mExtAddress));
+    mExtAddress.Clear();
 }
 
 otRadioCaps SubMac::GetCaps(void) const

src/core/thread/mle_router.cpp

@@ -2609,8 +2609,8 @@ void MleRouter::SetSteeringData(const Mac::ExtAddress *aExtAddress)
     Mac::ExtAddress nullExtAddr;
     Mac::ExtAddress allowAnyExtAddr;
 
-    memset(nullExtAddr.m8, 0, sizeof(nullExtAddr.m8));
-    memset(allowAnyExtAddr.m8, 0xFF, sizeof(allowAnyExtAddr.m8));
+    nullExtAddr.Clear();
+    allowAnyExtAddr.Fill(0xff);
 
     mSteeringData.Init();