ALSA: scarlett2: Change num_mux_* from int to u8

num_mux_srcs and num_mux_dsts will fit into a u8, so change the type.
More similar counts are coming soon.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/886fbd9ce7f06b13c6dbf36f64e6b2d107d16a83.1703444932.git.g@b4.vu


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