net: ipc_router: Fix mhi communication issue

MHI transport interface open got stuck due to deadlock
in open worker because of wrong usage of synchronization
object used in transport open path.

Fix the deadlock by signalling the flag from open worker
before calling hello send.

Change-Id: Ic65ce8975d34ae3c8c9efa69eae6a581cafbe4e9
Signed-off-by: Arun Prakash <app@codeaurora.org>