mhi: Do not free tre_ring memory multiple times

During device cleanup it is observed that tre_ring memory is freed
more than once. Kernel issues a warning on dmesg for such multiple
frees and it may result into undesirable results.

Hack the code to avoid such multiple frees.

Change-Id: I17d0a27b4d31d4a03fd56775a9a17d6623826b39
Signed-off-by: Pranjal Ramajor Asha Kanojiya <pkanojiy@codeaurora.org>