caif: Use RCU and lists in cfcnfg.c for managing caif link layers

RCU lists are used for handling the link layers instead of array.
When generating CAIF phy-id, ifindex is used as base. Legal range is 1-6.
Introduced set_phy_state() for managing CAIF Link layer state.
Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>