sound: Sort channels by unit number as well

[christos]

Summary

This makes the channel list easier to parse, as the channels are also
sorted by unit number.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days

[emaste]

What happens when (y)->type == t->type

[dev_submerge.ch]

sys/dev/sound/pcm/channel.h

227–237

This does not sort correctly. It can prematurely break out of the loop as soon as (y)->unit w t->unit isn't satisfied, thus inserting the channel among the wrong channel type. You probably want to prioritize one partial order (type) over the other (unit).

[christos]

Fix.

[dev_submerge.ch]

I think this is correct, but overly complicated. The classic sorting comparison would be:

if (((y)->type w t->type) || 
    (((y)->type == t->type) && ((y)->unit w t->unit)))
        a = t;
else
        break;