snd_hdsp(4): Support AO4S-192 and AI4S-192 extension boards.

Create an additional 4 channel pcm device for RME HDSP 9632 sound cards,
to support the optional AO4S-192 and AI4S-192 extension boards. For
simplicity, the <HDSP 9632 [ext]> pcm device is always present, even if
the extension boards are not installed.

Unfortunately I cannot test this with actual hardware, but I made sure
the additional channels do not affect the functionality of the HDSP 9632
as currently in src.

Reviewed by: christos, br
Differential Revision: https://reviews.freebsd.org/D46837