s390/qeth: convert bridgeport callbacks

By letting the callbacks deal with error translation, we no longer need
to pass the raw error codes back to the originator. This allows us to
slim down the callback's private data, and nicely simplifies the code.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>