kingst-la2016: more checks on configured rate/depth/channels

kingst-la2016: more checks on configured rate/depth/channels

Extend the list of supported samplerates, start from 10kHz (used to
start at 20kHz before). Comment on hardware constraints which limit
the available rates, for awareness during maintenance.

Move the check for enabled channels from api.c to protocol.c, it was
only used for the trigger configuration. Rename the routine since it
never configured any channels, only got the list of currently enabled
channels to pass on to the device. Extend the range checks for the
rate and depth on acquisition start, also check the lower limits.

Moving codes which references declarations required that definitions
also moved to public location of greater scope. This is acceptable since
all of them remain local to the kingst-la2016 driver. Stick with names
as they are used right now to reduce the diff and help reviewers, these
can get unified in a later commit.