ALSA: scarlett2: Check presence of mixer using mux_assignment

Currently the presence of a mixer is determined by checking if the
device uses the GEN_3A config set. Add scarlett2_has_mixer() function
which checks for the presence of mux_assignment entries instead.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/ef6f4d360c2fe682ab65f83cccbe5be66ccc6296.1703444932.git.g@b4.vu


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