ALSA: usb-audio: scarlett2: Remove hard-coded USB #defines

Remove the hard-coded interface number and related constants for the
vendor-specific interface and look them up from the USB endpoint
descriptor.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/20210620164652.GA9237@m.b4.vu


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