ALSA: hda/realtek - ALC891 headset mode for Dell

New headset mode of ALC891 for Dell.
This patch is supported Dell headset mode for ALC891.
It is only support I-phone type headset.
I think this function is only support for DELL.
This patch is test passed by Ubuntu team.

Signed-off-by: Kailang Yang <kailang@realtek.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>