usb: audio-v2: add ability to define feature unit descriptor

Similar to UAC1 spec, UAC2 feature unit descriptor
has variable size.

Current audio-v2 feature unit descriptor structure
is used for parsing descriptors, but can't be used
to define your own descriptor.

Add a new macro similar to what audio v1 already has.

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20210712125529.76070-2-pavel.hofman@ivitera.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>