kingst-la2016: adjust config param checks, threshold range check

kingst-la2016: adjust config param checks, threshold range check

After scan() completed the conn= can no longer be unknown. The firmware
was loaded and became effective (and communication was tested) before we
get to the config getter. Check PWM period and duty cycle value ranges
in the config setter.

Use symbolic names in the threshold voltage range check when protocol.c
forwards the configured value to the device.