ALSA: scarlett2: Split direct_monitor out from monitor_other

The notification value for monitor_other on the large interfaces is
the same as the notification value for direct_monitor on the 3rd Gen
small interfaces. Add a separate scarlett3a_notifications array and
split out the direct_monitor handling.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/9b56a483e3e9c1447684f18239a88652c1f01445.1703444932.git.g@b4.vuSigned-off-by: Takashi Iwai <tiwai@suse.de>