s390/qeth: remove a redundant check for card->dev

smatch complains that __qeth_l3_set_offline() first accesses card->dev,
and then later checks whether the pointer is valid.
Since commit d3d1b205 ("s390/qeth: allocate netdevice early"), the
pointer is _always_ valid - that patch merely missed to remove this one
check.

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