diag: Do not write buffer to usb when already flushed

Currently while draining the apps data, diag apps buffer is
queued to usb again which was already queued while processing
diag apps data. This race condition is resulting as use-after-free
since apps buffer will be freed on write done completion. The patch
checks flushed status of buffer before submitting it to usb.

Change-Id: I19d99958a14ae9a8483457a5a90c78d527d37fb0
Signed-off-by: Hardik Arya <harya@codeaurora.org>