net: ipc_router: Register services only on client port

Allowing services to be registered on a non-client port will cause either
an existing service or a control port to be over-written. This will cause
undefined functional behavior.

Allow the services to be registered only on client ports.

CRs-Fixed: 1101792
Change-Id: If6cfc75e9314204b7b44957f1598a8a2e1a45325
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>