diag: Flush control workqueue immediately on channel close

Currently, flushing of control workqueue is happening
under protection which is causing a deadlock. The patch
fixes the issue by flushing the control workqueue on
immediate closure of channel.

CRs-Fixed: 2081948
Change-Id: I6a7b1ee7cbabf2974700e28fc62c6d8fa3d464ed
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>