driver core: fix using 'ret' variable in unregister_dynamic_debug_module

The 'ret' variable is assigned, but not used in the return statement. Fix this.

Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
Acked-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>