ALSA: hda - Add missing hda_gen_spec to struct via_spec

The commit [4b527b65 ALSA: hda - limit internal mic boost for Asus
X202E] introduced the use of auto-parser code, but it forgot to add
struct hda_gen_spec at the head of codec->spec which the auto-parser
assumes silently.  Without this record, it may result in memory
corruption.

This patch adds the missing piece.

Cc: <stable@vger.kernel.org> [v3.5+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>