soc: qcom: bam_dmux: Do not perform close() on non-open channels

Performing a close operation on a channel that isn't opened may corrupt
internal state under some situations such as SSR.  Validate the channel
before allowing a close operation on it.

Change-Id: Ia8632163636e6ed08b2f68b98ba19d61c8a449fa
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>