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>