iio: adis1620x: Fix mixed up device descriptions

The module descriptions for the ADIS 16201, 16203 and 16209 drivers do not
match the actual function of the devices. Update them accordingly to fix
this.

Signed-off-by: Reto Schneider <code@reto-schneider.ch>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>