sysclk-lwla: Implement config_probe_set() callback.

sysclk-lwla: Implement config_probe_set() callback.

Move setup of channels and trigger masks to the new probe
configuration callback.  Although the actual hardware setup
still happens just before acquisition, the new approach
already has the advantage that invalid settings are caught
early.
Also, it turns out that the LWLA1034 allows triggering on
channels which are not enabled for data acquisition.  This
feature is now supported as well.