smcinvoke: Move IPC_LOG before deleting cb_txn

Move IPC_LOG inside mutex lock and bfefore deleting
cb_txn inside process_tzcb_req to make sure that
we are not accessing cb_req value that has gone bad.

Change-Id: Icc04bab52d1364d883b0a4bb07ff4cf2cc8ce6f8
Signed-off-by: Anmolpreet Kaur <anmolpre@codeaurora.org>