asix-sigma: keep remaining samplerate handling in protocol.c

asix-sigma: keep remaining samplerate handling in protocol.c

Make the list of supported samplerates an internal detail of the
protocol.c source file. Have the api.c source file retrieve the list
as well as the currently configured value by means of query routines.

Ideally the current rate could get retrieved from hardware at runtime.
A future driver implementation could do that. This version sticks with
the lowest supported rate, as in the previous version.