staging: iio: adis16300 add _type attributes for all scan elements

Note some of the in driver type specifications do not match the
data sheet.  I am assuming the datasheet is correct. This bug
would not have been apparent in the driver as these values were
unusued.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>