s390/net: fix format string mismatches

cppcheck blamed some issues in drivers/s390/net/...
They are fixed here.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Reported-by: Toralf Foerster <toralf.foerster@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>