asix-sigma: Properly decode data gathered at 100 and 200 MHz

asix-sigma: Properly decode data gathered at 100 and 200 MHz

The hardware provides captured data at a maximum rate of 16bits per 20ns
(50 MHz). For samplerates of 100 and 200 MHz one individual 16bit entity
contains multiple samples for a reduced number of channels. The bits of
several sample points are interleaved within the 16bit entity.

This commit is based on work done by jry@ who fixed a lot of issues with
the help from Ondrej at Asix.

This fixes bug #840.