net: ipc_router: Fix hang issue on xport init

If transport interface pass rx packet to ipc router
immediately after transport open there might be a
chance of calling sleep API which will cause hang issue
if the rx packet is coming from interrupt context.

To avoid this issue make sure transport registration
finished before exiting from transport open call to
avoid calling sleep API.

Change-Id: I0a61f7b9e0a450a6576d9289c599ba4a2d750371
Signed-off-by: Arun Prakash <app@codeaurora.org>