ALSA: hda/realtek - There is no loopback mixer in the ALC234/274/294

We have a Lenovo machine with the codec ALC294 on it, without the
patch, the Node 0x0b is regarded as the loopback mixer, but the Node
0x0b in this codec is "Vendor Defined Widget" instead of the audio
mixer, please see the log below:

Node 0x0b [Vendor Defined Widget] wcaps 0xf00000: Mono
  Control: name="Beep Playback Volume", index=0, device=0
    ControlAmp: chs=3, dir=In, idx=4, ofs=0
  Control: name="Beep Playback Switch", index=0, device=0
    ControlAmp: chs=3, dir=In, idx=4, ofs=0

And I have consulted with Realtek, so far the ALC234/274/294 all don't
have loopback mixer.

Cc: Kailang Yang <kailang@realtek.com>
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>