staging:iio:trivial add a string array for IIO_RAW and IIO_PROCESSED.

Introduce IIO_RAW and IIO_PROCESSED and an array for their strings.
Also move one function to just above where it is called.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>