aci: don't reset audio volume on AT+CFUN TI's AT-command-controlled modems have always had the peculiar misfeature where the audio volume gets reset to default on AT+CFUN, blowing away any AT+CLVL changes made previously. This misfeature is now getting worse, now that the audio volume may be set as a side effect of audio mode loading, in addition to explicit AT+CLVL. So let's take out this unnecessary audio volume reset.