s390/s390dbf: use debug_level_enabled() where applicable

Refactor direct debug level comparisons with the (internal) s390db->level
member.  Use the debug_level_enabled() function instead.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>