s390/qeth: remove gratuitous NULL checks

qeth_do_ioctl() is only reached through our own net_device_ops, so we
can trust that dev->ml_priv still contains what we put there earlier.

qeth_bridgeport_an_set() is an internal function that doesn't require
such sanity checks.

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