diag: Fix issues when memory-device mode closes

Diag driver is not cleaning up the buffers properly when the logging
mode is switched from memory-device mode to USB mode. Changing the
clean up sequence solves this issue.

Change-Id: I3523748a79e37489c378393f578f54d0a44bd0b9
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>