staging:iio:accel:adis16203 move to chan_spec based setup.

Mainly motivated by wish to remove the remaing users of the
scan helpers.

Some minor cleanups done whilst here.

Untested.

V2: Use IIO_CHAN macro
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>