iio: light: Simplify the current to lux LUT

The look-up-table for currens to lux is simply specifying
all currents from 5..47 mA, if we add some values for 0..5
we can just select the index for the lux value in an array
from the mA value. Use clamp() to get the value in the
range of values in the array.

Cc: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Gregor Riepl <onitake@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>