kingst-la2016: rephrase samplerate list code path, prepare 500MHz

kingst-la2016: rephrase samplerate list code path, prepare 500MHz

Rephrase the config_list() SR_CONF_SAMPLERATE branch for improved
readability. A list of supported rates gets derived from the current
device's maximum rate. Naming the tables after device models instead
of maximum rates is unexpected.

Drop the "odd" looking 4/8MHz items. Which results in consistent 1/2/5
steps in all decades. With hardware compression there is no gain to be
expected from using 4/8MHz instead of 5/10MHz. (Vendor software does
not provide that choice either.)

Add a list of samplerates for 500MHz models. These won't provide the
200MHz value, because it cannot result from the device's base clock
and an integer divider. Needs to be 250MHz instead. This is untested,
needs to get verified when a 500MHz model will be supported.