soc: qcom: ssr: Initialize the shutdown_ack completion variable properly

The shutdown_ack completion varaible, used during a call to
sysmon_send_shutdown(), is not initialized correctly. Initialize
it first while a subsystem regsiters with the SSR framework, followed
by a reinitialization before every wait_for_completion(). This gives
the flexibility for any subsystem that registers with the SSR
framework to make a call to sysmon_send_shutdown.

Change-Id: Ib28b9e27916bfb5ba41c82dae37e733e439eb4ee
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>