iio: light: ltr390: Calculate 'counts_per_uvi' dynamically

counts_per_uvi depends on the current value of gain and resolution.
Hence, we cannot use the hardcoded value 96.
The `counts_per_uvi` function gives the count based on the current gain
and resolution (integration time).

Signed-off-by: Abhash Jha <abhashkumarjha123@gmail.com>
Link: https://patch.msgid.link/20240814113135.14575-3-abhashkumarjha123@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>