[posix-host] update the function IsSafeToHandleNow() to avoid re-entrant issue (#4407)

mWaitingKey != SPINEL_PROP_LAST_STATUS means the NCP is waiting for a
spinel response from RCP. When mWaitingKey is set to
SPINEL_PROP_LAST_STATUS and HdlcInterface receives a ChildIdResponse
message from RCP，HdlcInterface calls the function Decode() to decode
the received HDLC frame. Then the decoded spinel frame is sent to
RadioSpinel module. Because the function IsSafeToHandleNow() returns
true now, the RadioSpinel sends the received ChildIdResponse frame to
up layer immediately. Then the MLE layer calls the function
otPlatRadioSetShortAddress() to set short address. The function
otPlatRadioSetShortAddress() sends spinel frame MAC_15_4_SADDR to RCP
and waits for the spinel response. When the response frame is
received, the HdlcInterface calls the function Decode() again, the
function Decode() uses previous calculated FCS value to calculate the
current received HDLC frame. The wrong FCS value caused the parsing
error for the received response frame.

src/posix/platform/radio_spinel.hpp

@@ -624,8 +624,7 @@ private:
      */
     bool IsSafeToHandleNow(spinel_prop_key_t aKey) const
     {
-        return !((mWaitingKey != SPINEL_PROP_LAST_STATUS) &&
-                 (aKey == SPINEL_PROP_STREAM_RAW || aKey == SPINEL_PROP_MAC_ENERGY_SCAN_RESULT));
+        return !(aKey == SPINEL_PROP_STREAM_RAW || aKey == SPINEL_PROP_MAC_ENERGY_SCAN_RESULT);
     }
 
     void HandleNotification(HdlcInterface::RxFrameBuffer &aFrameBuffer);