ALSA: hda/realtek - Re-add the model string selection for ALC88x

In the commit [c3e837bb

: ALSA: hda/realtek - Rewrite ALC882 acer-aspire-*
models with the auto-parser], the check of the model option got removed
mistakenly.  Re-added the board_config check again.

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