Address TEMPORARY_RETURN_IGNORED comments for silabs

[gmarcosb]

Gemini made several critical suggestions in #42089 of which many seem accurate, for example:

Ignoring the return value of GetPrimary802154MACAddress is dangerous. If this function fails, ifp->MacAddress will contain uninitialized data. The subsequent line ifp->hardwareAddress = ByteSpan(ifp->MacAddress, kMaxHardwareAddrSize); will then read from this uninitialized memory, leading to undefined behavior and a potential information leak vulnerability. The error must be checked and propagated.

    ReturnErrorOnFailure(ThreadStackMgrImpl().GetPrimary802154MACAddress(ifp->MacAddress));

Originally posted by @gemini-code-assist[bot] in #42089 (comment)

[gmarcosb]

Note that #42082 also contained some silabs suggestions