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

RME HDSPE AIO 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.

Test Plan:
The following tests on hardware should be performed if possible:

• Phones output volume changes according to sysctl setting (caution, still extremely loud).
• Create loop to feedback the line output to line input, recording the sound played.
• 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/D47412