iio: imu: inv_mpu6050: Use devm_iio_device_alloc

Using devm_iio_device_alloc makes code simpler. While at
it include missing iio.h header file and do some cleanup.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>