staging:iio:adc:ad7476 use channel_spec

V3: Trivial rebase fixup.
V2: Move to new single IIO_CHAN macro.

Done without hardware.

Fix from Michael Hennerich incorporated to use
iio_ring_buffer_register_ex instead of
iio_ring_buffer_register and thus actually make it work.

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