mailbox: msm_qmp: Do not reset channel open state

Only set the channel state to local opening on the first mailbox open.
All subsequent mailbox startup callbacks should check the completion
variable to see if the channel is already fully opened.

Change-Id: I76bc1f9344ff68c15c92992624dce937ecddda40
Signed-off-by: Chris Lew <clew@codeaurora.org>