pcm config files cleanups and name hint extension

- remove device 0/subdevice 0 from configuration files (it's default)
- name_hint
  - fixed parsing slaves
  - obtain device numbers directly from 'type hw' configurations to
    avoid poluting of configurations scripts with hint.device lines

src/conf/pcm/iec958.conf

@@ -78,5 +78,6 @@ pcm.!iec958 {
 			name defaults.namehint.basic
 		}
 		description "IEC958 (S/PDIF) Digital Audio Output"
+		device $DEV
 	}
 }