ALSA: usb-audio: scarlett2: Rename struct scarlett2_mixer_data

Rename struct scarlett2_mixer_data to struct scarlett2_data. A
less-wordy name is better because it is used everywhere, and although
this is a mixer driver, it also controls other vendor-specific
features.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/20210620164639.GA9206@m.b4.vuSigned-off-by: Takashi Iwai <tiwai@suse.de>