Always write the new mode to the sensor

In FORCE mode, the sensor changes back to SLEEP after completeing a single
measurement, but we are not updating the mode internally.  It's better to
always write the mode to the sensor, and trust the caller not to change it
needlessly.

Author: jraber

adafruit_bme280.py

@@ -198,8 +198,6 @@ def mode(self):
     def mode(self, value):
         if not value in _BME280_MODES:
             raise ValueError('Mode \'%s\' not supported' % (value))
-        if self._mode == value:
-            return
         self._mode = value
         self._write_ctrl_meas()