caif: prepare support for namespaces

Use struct net to reference CAIF configuration object instead of static variables.
Refactor functions caif_connect_client, caif_disconnect_client and squach
files cfcnfg.c and caif_config_utils.

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