msm: adsprpc: Wait for actual shutdown to complete

Wait for shutdown to complete, before further unmapping
memory in release.

Change-Id: Ic574a3e851f99aa946b2e062fa7a752c64b9f2b2
Acked-by: Deepika Singh <dsi@qti.qualcomm.com>
Signed-off-by: Vamsi Krishna Gattupalli <quic_vgattupa@quicinc.com>