usb: audio-v2: Add uac2_effect_unit_descriptor definition

The UAC2 Effect Unit Descriptor has a slightly different definition
from other similar ones like Processing Unit or Extension Unit.
Define it here so that it can be used in USB-audio driver in a later
patch.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200213112059.18745-2-tiwai@suse.de


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