ALSA: hda/realtek - Use common helper for creating ALC268 beep controls

The beep mixer controls are the only remaining stuff that uses
spec->mixers[] array, and they can be well converted to the standard
helper in the generic parser, snd_hda_gen_add_kctl().

This simplifies the code, especially the superfluous mixers and
num_mixers fields can be now removed from alc_spec.

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