drivers: nrf_clock_calibration: wait for sensor initialization

When using the RC clock source a periodic calibration is invoked that
involves reading from the die temperature sensor.  The code did not
protect against execution order that caused the periodic calibration
to be invoked before the temperature sensor was initialized.

Update the temperature sensor to detect that it has not been
initialized and so should reject attempts to fetch a reading.

Update the calibration code to do nothing when temperature reading
fails.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>