mailbox: msm_qmp: Fix qmp shutdown logic

If a mailbox fails to initialize correctly during the connecting stage,
the mailbox framework will call shutdown. The current logic incorrectly
increments ref counts when shutdown is called on a failing mailbox. Add
the correct handling for the connecting stages.

Change-Id: I9314aed833a9eecaaff9860e3261e89f6a7aa9c7
Signed-off-by: Chris Lew <clew@codeaurora.org>