iio: imu: inv_mpu6050: Remove casting the return value which is a void pointer

Casting the return value which is a void pointer is redundant.
The conversion from void pointer to any other pointer type is
guaranteed by the C programming language.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>