staging:iio:buffering cleanup ring_buffer_register_ex naming.

Now the old method is long gone, lets get rid of the _ex and whilst
here remove the unused id parameter.

Trivial mechanical change, but will break any out of tree drivers
using this.

V2: rebase
V3: rebase

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