iio: light: as73211: use IIO_VAL_FRACTIONAL for intensity scales

iio: light: as73211: use IIO_VAL_FRACTIONAL for intensity scales

The scale values associated to the light channels are calculated as a
division that can be better expressed as an IIO_VAL_FRACTIONAL type
instead of the current IIO_VAL_INT.

Note that the constant values used for the calculation were scaled up to
work with integers, turning the nW/cm^2 units from the datasheet into
nW/m^2, which would not be necessary with the IIO_VAL_FRACTIONAL type.
But to avoid issues from current users of the driver, the units must be
kept.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>