diag: Do not mark buffers free during channel opening

Currently, when launching mdlog session, buffers are being
marked free due to which file list command response is
getting corrupted leading to file list command timeout
error. This patch fixes the issue by keeping the buffers
busy which avoids queuing of next read.

CRs-Fixed: 2023646
Change-Id: I4db71c144875112a5ce24729b6b781488575a271
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>