hwdriver: Introduce sr_config_commit() API call.

(sr_dev_driver.config_probe_set): New optional callback enabling
drivers to be notified upon changes to probe settings.
(sr_dev_probe_enable, sr_dev_trigger_set): Invoke new driver
callback on changes.
(sr_dev_driver.config_commit): New optional callback allowing
drivers to defer application of configuration settings until
an explicit call to config_commit().
(sr_config_commit): New public wrapper function.
ols: Add option to turn test patterns off again.
rigol-ds: DS1000 series still needs the stupid delay.