iio:light:ltr501: Drop unnecessary cast of parameter in regmap_bulk_read

This only occurs once in the driver and isn't needed in this case either,
so drop it.

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>