rilmodem/call-volume: Fix parsing of SET_MUTE reply.

Based on commit '47ef2123723f347994e34a8e70c1183e38c3ce97' from
branch 'master' of https://github.com/nemomobile-packages/ofono.

Original-Author: Jarko Poutiainen <Jarko.Poutiainen@oss.tieto.com>