drivers/s390/net: Kconfig brush up

adapt drivers/s390/net/Kconfig to current IBM wording
and further cosmetics

Signed-off-by: Peter Tiedemann <ptiedem@de.ibm.com>
Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>