sensor: bme280: only configure mode on PM suspend/resume

Update power management to only start/stop periodic measurements in
`CONFIG_BME280_MODE_NORMAL`, instead of re-initialising the chip
completely.

In `CONFIG_BME280_MODE_FORCED`, there is nothing to do when suspending,
as the sensor is already in its lowest power mode.

Signed-off-by: Jordan Yates <jordan@embeint.com>