ALSA: usb-audio: Add audigy2nx resume support

Rewrite the code to handle LEDs on audigy2nx and co for supporting the
proper resume.  A new internal helper function
add_single_ctl_with_resume() is introduced to manage the
usb_mixer_elem_list more easily.

Also while we're at it, move audigy2nx_leds[] in usb_mixer_interface
struct into the private_value of each kctl, too.

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

sound/usb/mixer.h

@@ -23,7 +23,6 @@ struct usb_mixer_interface {
 	struct usb_ctrlrequest *rc_setup_packet;
 	u8 rc_buffer[6];
 
-	u8 audigy2nx_leds[3];
 	u8 xonar_u1_status;
 };