drivers: bme280: fetch data after measurement is done

After powering-on the sensor, and before every measurement, it loads
the NMV. We must wait until this process is completed otherwise
we will read weird values.
Since it was observed that the time may be a bit long after a cold
start, it is more convinient to just wait until the sensor iready,
without a timout.

Signed-off-by: Efrain Calderon <efrain.calderon@aquarobur.com>