s390/Kconfig.debug: fix indentation

The convention for indentation seems to be a single tab. Help text is
further indented by an additional two whitespaces. Fix the lines that
violate these rules.
Signed-off-by: Juerg Haefliger <juerg.haefliger@canonical.com>
Link: https://lore.kernel.org/r/20220525120151.39594-1-juerg.haefliger@canonical.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>