iio: chemical: bme680: Add check for val2 in the write_raw function

val2 is responsible for the floating part of the number to be
written to the device. We don't need the floating part
while writing the oversampling ratio for BME680 since the
available oversampling ratios are pure natural numbers.

So, add a sanity check to make sure val2 is 0.

Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>