kingst-la2016: cope with 800MHz base clock for 500MHz models

kingst-la2016: cope with 800MHz base clock for 500MHz models

The LA5016 uses an 800MHz base clock to derive the samplerate from. And
communicates divider 1 to configure the 500MHz rate. And does support
the 200MHz rate (divider 4). Which also makes the 10kHz lower boundary
unavailable on some models. Update the capture configuration logic, the
config API routines, and associated comments. Discuss how streaming may
make finer grained rates tables desirable (but stick with 1/2/5 for now).