caif: fix two caif_connect() bugs

caif_connect() might dereference a netdevice after dev_put() it.

It also doesnt check dev_get_by_index() return value and could
dereference a NULL pointer.

Fix it, using RCU to avoid taking a reference.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>