ALSA: usb-audio: Add support for the Pioneer DJM 750MK2 Mixer/Soundcard

The kernel already has support for very similar Pioneer djm products
and this work is based on that.

Added device to quirks-table.h and added control info to
mixer_quirks.c.

Tested on my hardware and all working.

Signed-off-by: William Overton <willovertonuk@gmail.com>
Link: https://lore.kernel.org/r/20211010145841.11907-1-willovertonuk@gmail.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>