ALSA: scarlett2: Rename scarlett_gen2 to scarlett2

This driver was originally developed for the Focusrite Scarlett Gen 2
series. Since then Focusrite have used a similar protocol for their
Gen 3, Gen 4, Clarett USB, Clarett+, and Vocaster series.

Let's call this common protocol the "Scarlett 2 Protocol" and rename
the driver to scarlett2 to not imply that it is restricted to Gen 2
series devices.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/e1ad7f69a1e20cdb39094164504389160c1a0a0b.1698342632.git.g@b4.vu


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