iio: chemical: bme680: Remove field value defines

Remove BME680_RUN_GAS_EN_BIT and BME680_NB_CONV_0_VAL field value
definitions because the fields are simply boolean and integer
respectively.

Signed-off-by: David Frey <dpfrey@gmail.com>
Reviewed-by: Himanshu Jha <himanshujha199640@gmail.com>
Tested-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>