msm: pil-q6v5-mss: Initialize the stop ack completion variable

In case of targets that don't use the PIL framework to boot the modem,
the stop_ack completion variable might remain in uninitialized state as
the initialization is currently being done only within PIL callbacks.
So, fix this my initializing the completion variable outside of PIL
by callbacks.

Change-Id: I204b0fbf461b1e1f1d4576dc43e33c7442eaf0e0
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>