diag: mhi: Remove unnecessary sanity check

On an MHI down notification, the enabled state is set to 0 before
flushing the workqueue and buffers. This sanity check conflicts
with the down notification. Remove unneeded sanity check.

Change-Id: I6bedc353187bee907775347009e0d4157b0e35e2
Signed-off-by: Chris Lew <clew@codeaurora.org>