msm: npu: Don't allow RSC handshake to be interrupted

If the RSC handshake is interrupted during NPU power down, it is
possible that NPUSS clocks will be turned off before it enters
power collapse mode which will put it into bad state.

Change-Id: Iae9023a8246b2816af341a9bd7234ae2b0ebf26e
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>