audio: fix cdc_uac2 speaker descriptor channel mismatch

The speaker Input Terminal declared 2 logical channels and the Feature Unit advertised master/ch1/ch2, while RX is 1, the AS interface declares 1 channel and mute[]/volume[] hold 2 entries (master + ch1). Align the speaker descriptors with the 1-channel config (mono). No change to the streaming format or EP size.