staging: iio: ad2s1200: Add scaling factor for angle channel

A scaling factor of approximately 2 * Pi / (2^12 -1) is added,
to scale the 12-bits angular position to radians.

A return type of IIO_VAL_INT_PLUS_NANO is used, so that the scale of
both the angle channel and angular velocity channel has 7 significant
digits.

Signed-off-by: David Veenstra <davidjulianveenstra@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>