msm: ADSPRPC: Validate channel status before write

Make sure that the channel is not closed before attempting
to write to the SMD channel.

Change-Id: I4ff4f57159c163ab4d330dbf4634ebf0b50931a2
Acked-by: Sathish Ambley <sambley@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>