staging:iio: fix removal path to allow correct freeing.

Fix a dumb lack of consideration of the effect of combining
the iio_device_unregister and iio_free_device calls into
one.  There is no valid place to free some of the sysfs
array elements.

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