tls: prevent recursive call to flushRxQueue()

TlsSession::flushRxQueue() let TlsSession::handleDataPacket() to be called
causing a recursive call to TlsSession::flushRxQueue(), finally resulting
into application crash.

Solve that by testing if we're in flush opeation and just no-op in such case.

Change-Id: Ie4d52a9df495784e36e3691404f44cd8f66dbb6d