caif: make zero a legal caif connetion id.

Connection ID configured through RTNL must allow zero as
connection id. If connection-id is not given when creating the
interface, configure a loopback interface using ifindex as
connection-id.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>