iio: light: tsl2591: fix some signedness bugs

These variables need to be int for the error handling to work.

Fixes: 2335f0d7 ("iio: light: Added AMS tsl2591 driver implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YJ52r1XZ44myD9Xx@mwanda


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