ALSA: hda/realtek - Support Dell headset mode for ALC256

Dell new platform of ALC256 audio codec.
Support headset mode for Dell ALC256 platform.

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