caif: Use rcu_read_lock in CAIF mux layer.

Replace spin_lock with rcu_read_lock when accessing lists to layers
and cache. While packets are in flight rcu_read_lock should not be held,
instead ref-counters are used in combination with RCU.

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