Disable Headset Mic VREF for headset mode of ALC225.
This will be controlled by coef bits of headset mode functions.

[ Fixed a compile warning and code simplification -- tiwai ]
Signed-off-by: Kailang Yang <kailang@realtek.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>