staging:iio:adc:ad7150: chan_spec conv + i2c_smbus commands + drop unused poweroff...

Minimal changes to code layout as going to do chan spec conversion shortly.
Otherwise, there are numerous code sharing opportunities in here and abi
elements that are uterly non compliant.

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>