dynamic_debug: early return if _ddebug table is empty

If _ddebug table is empty (in a CONFIG_DYNAMIC_DEBUG build this
shouldn't happen), then warn (error?) and return early.  This skips
empty table scan and parsing of setup-string, including the pr_info
call noting the parse.  By inspection, copy return-code handling from
1st ddebug_add_module() callsite to 2nd.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>