staging:iio:adc:ad7291 bring into line with current abi + chan_spec conversion.

Also add some locking.

Some major changes to how this driver works.

For voltage channels it is currently either in single read mode or in
a monitor mode (events only). Could be much cleverer and allow
reading of any channels that happend to be monitored, but haven't
done that yet.
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>