msm: subsys-pil-tz: Correct initializing of stop_ack completion struct

The stop_ack completion struct is initialized after subsys_register
is called. This might lead to a race condition where the stop_ack
interrupt might be received before the stop_ack completion struct
is initialized.
Move the initialization to before the call to subsys_register is made.

Change-Id: I47db9fd892150deff317a536f086062fb324000d
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>