`rtc_write` must start the RTC. `time()` uses `rtc_write(0)` to start
the RTC if it is not already enabled, but this check made that not
work.

There's no point trying to optimise this case in a HAL - if we wanted
`set_time()` to protect against users making pointless adjustments, the
implementation should be there. But even then, you might want different
levels of hysteresis depending on application, so it's probably best
left to applications.

This reverts commit 9da5e48.