iio: common: scmi_sensors: Get rid of const_ilog2()

iio: common: scmi_sensors: Get rid of const_ilog2()

Fisrt of all, const_ilog2() was a workaround of some sparse issue,
which was never appeared in the C functions. Second, the calls here
are done against constants and work with a bit of luck. Replace
this altogether by a pre-calculated simple integer constant.
Amend a comment to give a hint where it comes from.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>