ASoC: bg: Fix incorrect num_session update

For SVA_usecase codec_prepare is not invoked which results in a
incorrect count of num_sessions. Update num_session in _bg_codec_start
instead, this ensures correct count of num_session.

Change-Id: I4ee5b15f969c02d3943b282203f569253a0b6499
Signed-off-by: Ashish Jain <ashishj@codeaurora.org>