ALSA: hda - Prefer VREF50 if BIOS sets for Realtek codecs

If BIOS sets up the input pin as VREF 50, use the value as is instead of
overriding forcibly to VREF 80.  This fixes the quality of inputs on
some devices like Packard-Bell M5210.

Signed-off-by: Takashi Iwai <tiwai@suse.de>