staging:iio: ABI rework - add in_ or out_ prefix to channnels

Also involves changing current inX outX to in_voltageX and out_voltageX

V2: squash users of the IIO_CHAN_OUT macro and get rid of it.
There are very few of these, so it is easier to fix them.

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