staging: iio: ad2s1200: Add scaling factor for angular velocity channel

The sysfs iio ABI states radians per second is expected as the unit for
angular velocity, but the 12-bit angular velocity register has
revolution per seconds as its unit. So a scaling factor of approximately
2 * Pi is added to the angular velocity channel.

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