kingst-la2016: rework the device side of PWM configuration

kingst-la2016: rework the device side of PWM configuration

Assign an initial PWM configuration during scan (arbitrary frequencies
at 50% duty cycle as before, but off by default). Ideally the initial
assignment gets updated by user specs at runtime, and hardware gets
configured as needed from internal data which was kept at defaults or
updated from user specs.

Combine the previous enable_pwm() and configure_pwm() routines into
set_pwm_config() which configures one channel in the device's hardware
(and transparently controls the enable flag). This eliminates the
unfortunate "channel unrolling" in the helper's API. Which eliminates
la2016_init_params() which redundantly configured the logic threshold
(done upon acquisition start again) and kept reassigning PWM parameters
across multiple open calls.

Providing the config API to adjust PWM settings remains to be done.