iio: gyro: itg3200_core: Use devm_iio_device_alloc

Using devm_iio_device_alloc makes code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Christian Strobel <christian.strobel@iis.fraunhofer.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>