dsp: call voice MHI start/end APIs from q6voice

Whenever a voice call starts or ends q6voice shall call
voice_MHI API to enable or disable MHI voting respectively.

Change-Id: Ia1125fb96c67ff6fec2242bc2b482ef7017dfb7d
Signed-off-by: Vikram Panduranga <vpandura@codeaurora.org>