hal: Avoid stereo device selection for target with single speaker

Stereo flag is true by default, speaker device in selction
of VOICE_SPEAKER_2 during call for target with single speaker.

Fix this by set stereo flag to false for target with single
speaker to ensure proper speaker device is selected.

CRs-Fixed: 1113317
Change-Id: I62d5b9cf84656272d93b2810b35a53b3e27465cd