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>