iio: bmp280: properly initialize device for humidity reading

If the device is not initialized at least once it happens that the humidity
reading is skipped, which means the special value 0x8000 is delivered.

For omitting this case the oversampling of the humidity must be set before
the oversampling of the temperature und pressure is set as written in the
datasheet of the BME280.

Furthermore proper error detection is added in case a skipped value is read
from the device. This is done also for pressure and temperature reading.
Especially it don't make sense to compensate this value and treat it as
regular value.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>