ALSA: usb-audio: Skip broken EU on Dell dock USB-audio

The Dell Dock USB-audio device with 0bda:4014 is behaving notoriously
bad, and we have already applied some workaround to avoid the firmware
hiccup.  Yet we still need to skip one thing, the Extension Unit at ID
4, which doesn't react correctly to the mixer ctl access.

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1090658
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_maps.c

@@ -353,8 +353,11 @@ static struct usbmix_name_map bose_companion5_map[] = {
 /*
  * Dell usb dock with ALC4020 codec had a firmware problem where it got
  * screwed up when zero volume is passed; just skip it as a workaround
+ *
+ * Also the extension unit gives an access error, so skip it as well.
  */
 static const struct usbmix_name_map dell_alc4020_map[] = {
+	{ 4, NULL },	/* extension unit */
 	{ 16, NULL },
 	{ 19, NULL },
 	{ 0 }