soc: qcom: Avoid race shutdown and receive data for mbox channel

In scenario where channel shutdown is in progress interrupt
handler can call callback for mbox channel which might be
free.

Change to avoid race between mbox channel free and receive
data.

Change-Id: I07ef5f08aa1946be775de6b3e2f89d9069fa6ee3
Signed-off-by: Mayank Grover <groverm@codeaurora.org>