ALSA: scarlett2: Add #define for SCARLETT2_MIX_MAX

Add a #define for SCARLETT2_MIX_MAX (max of mixer inputs * outputs) as
that will be used again soon.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/83cec5ccd75f0db2bd061a76d31a7023d26300c1.1703444932.git.g@b4.vu


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