ALSA: MIDI driver for Behringer BCD2000 USB device

This patch adds initial support for the Behringer BCD2000 USB DJ controller.
At the moment, only the MIDI part of the device is working, i.e. knobs,
buttons and LEDs.

I also plan to add support for the audio part, but I assume that this will
require more effort than the rather simple MIDI interface. Progress can be
tracked at https://github.com/anyc/snd-usb-bcd2000.

Signed-off-by: Mario Kicherer <dev@kicherer.org>
Reviewed-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>