iio: magnetometer: hid-sensor-magn-3d: Use devm_iio_device_alloc

Using devm_iio_device_alloc makes code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Srinivas Pandruvada <srinivas.pandruvada@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>