extmod/modussl_mbedtls: Make socket.close() free all TLS resources.

Also, use mp_stream_close() helper to close the underlying socket.

extmod/modussl_mbedtls.c

@@ -227,15 +227,15 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(socket_setblocking_obj, socket_setblocking);
 STATIC mp_obj_t socket_close(mp_obj_t self_in) {
     mp_obj_ssl_socket_t *self = MP_OBJ_TO_PTR(self_in);
 
+    mbedtls_pk_free(&self->pkey);
+    mbedtls_x509_crt_free(&self->cert);
     mbedtls_x509_crt_free(&self->cacert);
     mbedtls_ssl_free(&self->ssl);
     mbedtls_ssl_config_free(&self->conf);
     mbedtls_ctr_drbg_free(&self->ctr_drbg);
     mbedtls_entropy_free(&self->entropy);
 
-    mp_obj_t dest[2];
-    mp_load_method(self->sock, MP_QSTR_close, dest);
-    return mp_call_method_n_kw(0, 0, dest);
+    return mp_stream_close(self->sock);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(socket_close_obj, socket_close);