caif: Remove bad WARN_ON in caif_dev

Remove WARN_ON and bad handling of SKB without destructor callback
in caif_flow_cb. SKB without destructor cannot be handled as an
error case.

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