staging: iio: fix access to hmc5843 private data

i2c_get_clientdata(client) points to iio_dev, not hmc5843_data; fixes
issue similar to 62d2feb9

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>