Staging: iio: adc: fix dangling pointers

Fix I2C-drivers which missed setting clientdata to NULL before freeing the
structure it points to. Also fix drivers which do this _after_ the structure
was freed already.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>