caif: Handle dev_queue_xmit errors.

Do proper handling of dev_queue_xmit errors in order to
avoid double free of skb and leaks in error conditions.
In cfctrl pending requests are removed when CAIF Link layer goes down.

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