iio: tsl2583: correct values in integration_time_available

The times reported by the in_illuminance_integration_time_available
sysfs attribute are actually in milliseconds, not microseconds. This
patch corrects the times with the correct unit.

The fixes tag is inaccurate as the issue existed when the driver
was still in staging.   However, lots of changes occured before
it graduated so this is as a good a point as any for backports.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Fixes: f44d5c8a ("staging: iio: tsl2583: move out of staging")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>