msm: ADSPRPC: Close SMD channel after RPC session ends

The SMD channel open request needs to be delayed as the
remote processor may not be up early on when the driver
is initialized. The SMD channel is opened when the first
RPC session is opened and closed when the last RPC
session ends.

Change-Id: I0bbff2956c3e367312d4344107340feb29e643ab
Acked-by: Sathish Ambley <sambley@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>