diag: Prevent memory leak when switching from Memory Device Mode

When diag switches from Memory device mode to any other mode,
buffers are not freed for Local Diag data. This can lead to memory
pool exhaustion resulting in Diag packet drops.

Change-Id: Icfeb00dd6c11091a7b9106fbefcdcc9d028782c8
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>