ALSA: hda - Fix silent output on ASUS A6Rp

The refactoring of Realtek codec driver in 3.2 kernel caused a
regression for ASUS A6Rp laptop; it doesn't give any output.
The reason was that this machine has a secret master mute (or EAPD)
control via NID 0x0f VREF.  Setting VREF50 on this node makes the
sound working again.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=42588

Cc: <stable@kernel.org> [v3.2+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>