slim_ngd: Use DSP before-shutdown notification for DSP SSR

QMI_SERVER_EXIT notification comes too late (after the DSP has
shutdown). This can result in unclocked access by BAM if there was
in-flight transaction. Use BEFORE_SHUTDOWN notification of SSR
framework instead to make sure in-flight transaction is finished
before DSP turns off its resources.

CRs-Fixed: 730552
Change-Id: If3c2787c74ffa7743772e5e37eac2f768e239050
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>
Signed-off-by: Dilip Kota <dkota@codeaurora.org>