diag: Set in_busy only after initialization of read buffers

Currently, there is a possibility that read buffers are
not initialized but diag ends up marking buffers busy and
stops reading from peripheral, which leads to port loss.
Also reset the in_busy variable for the buffer that reads
the data from smd channel when diag smd state is closed.

CRs-Fixed: 1093989
Change-Id: I1990d0ded5a212c9185c149ac297a3630d64bc59
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>