asix-sigma: concentrate parameters for sample memory interpretation

Create a sub struct in the device context which keeps those parameters
which are related to sample memory interpretation. Which also obsoletes
the 'state' struct and only leaves the 'state' enum as a remainder.

Use the "samples per event" condition instead of the samplerate when
extracting a number of samples from an event's storage. Rename the
de-interleaving routines to better reflect their purpose.