hal: start keep alive session to feed EC with silence data

FFV EC reference signal is fed from AFE RX port.
Start the keep alive session to write silence data to
RX port and feed the silence data to FFV EC.

Change-Id: I38b0396ef853493c9a176444dc803744be1c45bb
Signed-off-by: Surendar karka <sukark@codeaurora.org>