asix-sigma: reword list of sample rates, (try to) use 1/2/5 steps

asix-sigma: reword list of sample rates, (try to) use 1/2/5 steps

The driver got extended in a previous commit to accept any hardware
supported samplerate in the setter API, although the list call does
suggest a discrete set of rates (a subset of the hardware capabilities).
Update a comment to catch up with the implementation.

Drop the 250kHz item, it's too close to 200kHz. Add a 2MHz item to
achieve a more consistent 1/2/5 sequence in each decade. Unfortunately
50MHz and an integer divider will never result in 20MHz, that's why
25MHz is an exception to this rule (has been before, just "stands out
more perceivably" in this adjusted sequence).