ALSA: snd-usb-caiaq: LED support for Maschine Controller

This patch adds LED support for the Native Instruments Maschine
Controller. It adds ALSA controls for dimming the LEDs of all
buttons and the backlight of the two displays.

Signed-off-by: Hannes Gräuler <hgraeule@uos.de>
Acked-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>