smcinvoke: Return proper error in process_accept_req

When the userspace sends a wrong transaction id or the
invoke threads have died for some reason, then there will
be no cb_txn in the responses table. In such case, we
should send a signal to userspace to send the thread again
so that it starts waiting for new callback requests. This
change adds the valid return value to be sent to userspace
to address the same.

Change-Id: I69fe45c3db8b1a0f44e43a9b926c42c73be6ffca
Signed-off-by: Anmolpreet Kaur <anmolpre@codeaurora.org>