ALSA: scarlett2: Infer standalone switch from config items

Rather than assuming the standalone switch is present for all devices
with a mixer, instead check for the presence of the
SCARLETT2_CONFIG_STANDALONE_SWITCH config item.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/59c30885b02d65feaab2c338cf46889d72d01813.1703444932.git.g@b4.vu


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