dev_dbg: check dev_dbg() arguments

Duplicate what Zach Brown did for pr_debug in commit
8b2a1fd1



[akpm@linux-foundation.org: fix a couple of things which broke]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>