logfs: do not use 'mtd->block_isbad' directly

Instead, use the new 'mtd_can_have_bb()' helper.

Cc: Jörn Engel <joern@logfs.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>