msm: rpm-smd: Handle multiple link up events

If multiple transports for an edge come up, rpm-smd can get multiple link
up events.  Rpm-smd will then attempt to incorrectly open the same channel
multiple times, which is expected to fail and could incorrectly trigger a
BUG_ON().

Fix rpm-smd to only open the channel when it gets the first link up event,
and ignore any additional link up events.

CRs-fixed: 763569
Change-Id: I1286a3fdb7d3d2f4dbfeaed3624e742ffcaf3a8c
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>