qeth: display service_level info only when valid.

qeth displayed an entry in /proc/service_level even when no valid
MCL-string was available (the MCL info is blank). The change is to
create an entry in /proc/service_level only when MCL-string is
non-zero.

Signed-off-by: Klaus-Dieter Wacker <kdwacker@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>