s390/qeth: use netdev_name()

Replace our custom version of netdev_name().

Once we started to allocate the netdev at probe time with
commit d3d1b205 ("s390/qeth: allocate netdevice early"), this
stopped working as intended anyway.

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