msm: ipc: Serialize SMD XPRT CLOSE and OPEN events

When a remote subsystem resets, IPC Router's SMD XPRT Layer queues a CLOSE
event. When the concerned subsytem powers up and the channel is reopened,
SMD XPRT Layer queues a OPEN event. When the remote subsytem resets and
powers up quickly, the following race condition exists:
a) IPC Router is scheduled out while handling the CLOSE event, leaving a
   stale XPRT
b) IPC Router's SMD XPRT Layer queues a OPEN event
c) IPC Router's SMD XPRT Layer reads a HELLO message, finds a stale
   XPRT and queues a DATA event on the stale XPRT
This race condition causes a kernel panic.

Fix this race condition, by not handling any other SMD XPRT event until
the CLOSE event is complete.

CRs-Fixed: 488331
Change-Id: I2283e163413d1896bd6842530b8a6a2b537e22de
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>