ALSA: snd-usb-caiaq: Add support for Traktor Kontrol S4

This patch adds support for the new Traktor Kontrol S4 by Native
Instruments. It features a new audio data streaming model, MIDI
in and out ports, a huge number of 174 dimmable LEDs, 96 buttons
and 46 absolute encoder axis, including some rotary encoders.

All features are supported by the driver now.

Did some code refactoring along the way.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>