iio: gyro: st_gyro: Use devm_iio_device_alloc

Using devm_iio_device_alloc makes code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>