When checking that a card has all of its PCM devices available, ignore
any specific device with the ACP_IGNORE udev environment variable. This
mirrors how we ignore whole cards, but specifically allows non-PipeWire
software to own specific PCM devices.
Note that this does not actually stop PipeWire from using those
subdevices right now, we assume UCM configs take care of that. This
should probably be implemented later to ensure PipeWire always stays
away from them, but for now this fixes the issue where it refuses to
probe the entire card.
Fixes: #3570
Signed-off-by: Hector Martin <marcan@marcan.st>