ALSA: hda/realtek - Fix missing inits of item indices for auto-mic
When the imux entries are rebuilt in alc_rebuild_imux_for_auto_mic(),
the initialization of index field is missing. It may work without it
casually when the original imux was created by the auto-parser, but
it's definitely broken in the case of static configs where no imux was
parsed beforehand. Because of this, the auto-mic switching doesn't
work properly on some model options.
This patch adds the missing initialization of index field.