snd_hdspe(4): One pcm device per physical ADAT port.

ADAT connections transport 8, 4 or 2 audio channels depending on the
sample rate. Instead of splitting each physical ADAT port into 4
(potentially unmapped) stereo pcm devices, create just one pcm
device of variable channel width for every ADAT port.
Depending on the sample rate and channel width selected, the pcm
channels may be only partially mapped to ADAT channels and vice versa.

Added flexibility of the new channel mapping is also prerequisite to
introduce more pcm device layouts in follow-up commits.

Reviewed by: br
Differential Revision: https://reviews.freebsd.org/D43393