ALSA: hda/realtek - Support ALC300

This patch will enable ALC300.

[ It's almost equivalent with other ALC269-compatible ones, and
  apparently has no loopback mixer -- tiwai ]

Signed-off-by: Kailang Yang <kailang@realtek.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>