ALSA: scarlett2: Check for phantom persistence config item

Allow for the phantom persistence config item to not exist. This is
needed for the Scarlett Gen 4 series.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/3ccaf8069280827bd6c44f103fcb770bd50b7e2e.1703444932.git.g@b4.vu


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