iio:chemical:bme680: Tidy up parameters to regmap_bulk_read

A mixture of:
* Unnecessary casts of val parameter to u8 * which makes little sense as
  the function take a void *.
* Explicit sizes where we can use the destination type to define that.
  Note that's not true in all cases as we do read 3 bytes into 4 byte
  destinations.

Note that noting was broken here, I'm just trying to ensure this doesn't
get cut and paste into more drivers so cleaning these out subsystem wide.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Himanshu Jha <himanshujha199640@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>