ALSA: usb/caiaq: Add support for Traktor Kontrol X1

This device does not have audio controllers and backlit buttons only.
Input data is handled over a dedicated USB endpoint.

All functions are supported by the driver now.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Takashi Iwai <tiwai@suse.de>