s390/qeth: avoid null pointer dereference on OSN

s390/qeth: avoid null pointer dereference on OSN

BugLink: http://bugs.launchpad.net/bugs/1697001
[ Upstream commit 25e2c341e7818a394da9abc403716278ee646014 ]

Access card->dev only after checking whether's its valid.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>