scarlett2_add_firmware_version_ctl() may return an error, but the
return value was not being checked. Add the missing check.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Fixes: 701949cc ("ALSA: scarlett2: Add support for reading firmware version")
Link: https://lore.kernel.org/r/ZTuKcXajVnuelBEb@m.b4.vuSigned-off-by: Takashi Iwai <tiwai@suse.de>