ALSA: usb-audio: Clean up mixer element list traverse

Introduce a new macro for iterating over mixer element list for
avoiding the open codes in many places.  Also the open-coded
container_of() and the forced cast to struct usb_mixer_elem_info are
replaced with another simple macro, too.

No functional changes but just readability improvement.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_quirks.c

@@ -1172,7 +1172,7 @@ void snd_emuusb_set_samplerate(struct snd_usb_audio *chip,
 	int unitid = 12; /* SamleRate ExtensionUnit ID */
 
 	list_for_each_entry(mixer, &chip->mixer_list, list) {
-		cval = (struct usb_mixer_elem_info *)mixer->id_elems[unitid];
+		cval = mixer_elem_list_to_info(mixer->id_elems[unitid]);
 		if (cval) {
 			snd_usb_mixer_set_ctl_value(cval, UAC_SET_CUR,
 						    cval->control << 8,