snd_hdsp(4): Add sysctls to select analog signal levels.

RME HDSP 9632 sound cards support different sensitivity levels at the
analog input, and different gain levels for the analog line and phones
outputs. Expose these settings as sysctl tunables.

Tested on hardware:

• Phones output volume changes according to sysctl setting (caution, still extremely loud).
• Create loop to feedback the line output to line input.
• Iterate through input signal levels and observe volume changes in recorded audio.
• Iterate through output signal levels and observe volume changes in recorded audio.

Reviewed by: br, christos
Differential Revision: https://reviews.freebsd.org/D47330