staging:iio:gyro:adis16260 move to chan_spec based setup.

For clean drivers its easier to convert to chan_spec based
registration than to work around the scan_el attributes
going away.

Some minor cleanups done whilst here.

Untested.
V3: rebase fixup and make attribute_group static.

V2: IIO_CHAN macro updates.

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