ALSA: scarlett2: Add #defines for firmware upgrade

Add #defines for SCARLETT2_USB_* needed for firmware upgrade:
reboot, info-flash, info-segment, erase-segment, get-erase, and
write-segment.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/3077651c21bc8d4f046c68b79ec387aa16fcc5e4.1703001053.git.g@b4.vu


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