[ALSA] patch_realtek.c: Add new model

Modules: HDA Codec driver

This little patch add the model for the motherboard K8N51 from Gigabyte
to the known models of ALC boards.

Signed-off-by: Arnaud Patard <apatard@mandriva.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>