diag: Do not close mhi channels on usb disconnect

Do not close mhi channels on usb disconnect. Close the
channels only when a ssr notification from MHI occurs.
This will reduce the over head of closing and opening
mhi channels during usb disconnect/connect. Also this
will resolve a deadlock issue while switching the
logging from memory device mode to usb.

Change-Id: If57bc0d4355da1f8e400bab8a312988721157eab
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>