iio:imu:mpu6050: Tidy up parameters to regmap_bulk functions.

A mixture of:
* Casts to u8 * for the val parameter which is void * and so needs no cast.
* Explicit sizes when we can use sizeof on the destination / source variable.

Not broken in anyway, I'm just trying to tidy up the is in the subsystem to
avoid it getting cut and paste into new drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>