iio: accel: bma400: prevent setting accel scale too low

This puts an upper bound on "val2" but it also needs to have a lower
bound (BMA400_SCALE_MIN).

Fixes: 465c811f ("iio: accel: Add driver for the BMA400")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>