ALSA: scarlett2: Add Focusrite Clarett+ 2Pre and 4Pre support

The Focusrite Clarett+ series uses the same protocol as the Scarlett
Gen 2 and Gen 3 series. This patch adds support for the Clarett+ 2Pre
and Clarett+ 4Pre similarly to the existing 8Pre support by adding
appropriate entries to the scarlett2 driver.

The Clarett 2Pre USB and 4Pre USB presumably use the same protocol as
well, so support for them can easily be added if someone can test.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/ZRL7qjC3tYQllT3H@m.b4.vu
Signed-off-by: Takashi Iwai <tiwai@suse.de>