[S390] css: fix sparse warning

fix this sparse warning:

drivers/s390/cio/css.c:580:6: warning: symbol 'css_schedule_eval_all_unreg'
was not declared. Should it be static?

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>