asix-sigma: Adjust clock configuration upon acquisition start

Configure the samplerate clock and channel count during acquisition
start in identical ways for 50MHz, 100MHz, and 200MHz modes.

This part was inspired by work done by jry@ yet was addressed in
different ways (no exception, do everything in every mode the same way).

Eliminate a portability issue in the previous implementation. Make sure
to send the configuration bytes in the correct order to the hardware.
Don't typecase a struct reference to a bytepointer and hope that the
internal memory representation might fit the external hardware's idea.