[cli] set mUseTls to false after de-initializing TCP session (#10977)

Avoid SSL context operations in TCP callbacks (for
eg. `HandleTcpDisconnected`) when the context could have already been
de-allocated after de-initialization.

src/cli/cli_tcp.cpp

@@ -304,6 +304,8 @@ template <> otError TcpExample::Process<Cmd("deinit")>(Arg aArgs[])
 
         mbedtls_pk_free(&mPKey);
         mbedtls_x509_crt_free(&mSrvCert);
+
+        mUseTls = false;
     }
 #endif