sensor: bme280: clamp humidity to non-negative

Clamp the returned humidity to a non-negative value. This ensure that
the return value is always within the expected range, even if the
calibration parameters are read out incorrectly or corrupted.

This check is applied in the example Bosch driver for the float
conversion case, so there are situations that warrant it.

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