kingst-la2016: use symbolic names for device run state

kingst-la2016: use symbolic names for device run state

Rephrase the run_state() routine in terms of symbolic identifiers,
eliminate magic numbers in the code. Extend comments on run state even
more to help reviewers. Reduce indentation in the optional logging when
the value changes.

It's assumed that the "current state and mask equals a typical pattern?"
phrase better reflects the purpose of a check, and is easier to verify
than hex numbers in a context where bit field meanings matter and not
all adjacent bits are involved).

This implementation no longer assumes that "sample to DRAM" is set. In
theory capture to internal RAM versus streaming via USB should not matter
when the acquisition state is queried.