ALSA: scarlett2: Refactor common port_count lookups

Rather than looking up the analogue and mixer I/O counts repeatedly in
info->port_count[SCARLETT2_PORT_TYPE_*][SCARLETT2_PORT_*], save those
numbers in private variables.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/db0a5b56bdff476e2e31ad8e5ee15008314412b7.1703444932.git.g@b4.vuSigned-off-by: Takashi Iwai <tiwai@suse.de>