[radio] avoid using zero-initializer lists for Mac types (#11921)

This change updates `Radio::Init()` to avoid passing
zero-initialized temporary objects of `Mac::ExtAddress` and
`Mac::KeyMaterial` types.

Instead, it declares local variables and passes them to the setter
methods. This improves code readability and makes the intent more
explicit.

src/core/radio/radio.cpp

@@ -48,6 +48,9 @@ const uint8_t Radio::kSupportedChannelPages[kNumChannelPages] = {
 void Radio::Init(void)
 {
 #if OPENTHREAD_CONFIG_RADIO_LINK_IEEE_802_15_4_ENABLE
+    Mac::ExtAddress  allZeroExtAddress;
+    Mac::KeyMaterial emptyKeyMaterial;
+
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
     SuccessOrAssert(ResetCsl());
 #endif
@@ -63,9 +66,11 @@ void Radio::Init(void)
     }
 
     SetPanId(Mac::kPanIdBroadcast);
-    SetExtendedAddress(Mac::ExtAddress{});
+    allZeroExtAddress.Clear();
+    SetExtendedAddress(allZeroExtAddress);
     SetShortAddress(Mac::kShortAddrInvalid);
-    SetMacKey(0, 0, Mac::KeyMaterial{}, Mac::KeyMaterial{}, Mac::KeyMaterial{});
+    emptyKeyMaterial.Clear();
+    SetMacKey(0, 0, emptyKeyMaterial, emptyKeyMaterial, emptyKeyMaterial);
     SetMacFrameCounter(0);
 
     SetPromiscuous(false);