sysclk-lwla: Implement SR_CONF_LIMIT_MSEC.

sysclk-lwla: Implement SR_CONF_LIMIT_MSEC.

Allow the acquisition to be constrained by time in addition to
a sample count limit.  Since the LWLA protocol actually provides
only a duration natively, implement the sample count limit on top
of the new duration limit.

With this change, limiting an acquisition in external clock mode
should finally work properly.