ALSA: scarlett2: Fix pad count for 18i8 Gen 3

The 18i8 Gen 3 has 4 inputs with a pad control, not 2. Update
s18i8_gen3_info.pad_input_count.

Reported-by: Aaron Wolf <aaron@wolftune.com>
Tested-by: Aaron Wolf <aaron@wolftune.com>
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/29a6ce412a42373daab7c96c395560461fcf08c6.1624798436.git.g@b4.vu


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