claw/lcs/netiucv: check s390dbf level before sprints

additional check of s390dbf level results in better performance
if the default low debugging level is active.

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>