diag: Flush the data only in case for Memory Device Mode

Once the logging is switched, from USB_MODE to MEMORY_DEVICE_MODE
diag is not expecting write done notification from USB. In case
for asynchronous writes notification for USB, the data can be
dropped. This ensures that the flush_workqueue is called only
in case where diag is logging in Memory device mode and write
done notification is being called from memory device context.

Change-Id: Ideed995f96797e1950647c3657e4ab013dda74aa
Signed-off-by: Katish Paran <kparan@codeaurora.org>