caif: Use RCU instead of spin-lock in caif_dev.c

RCU read_lock and refcount is used to protect in-flight packets.

Use RCU and counters to manage freeing lower part of the CAIF stack if
CAIF-link layer is removed. Old solution based on delaying removal of
device is removed.

When CAIF link layer goes down the use of CAIF link layer is disabled
(by calling caif_set_phy_state()), but removal and freeing of the
lower part of the CAIF stack is done when Link layer is unregistered.

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