net: caif: replace BUG_ON with recovery code

In caif_xmit, there is a crash if the ptr dev is NULL. However, by
returning the error to the callers, the error can be handled. The
patch fixes this issue.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>