staging: iio: tsl2583: change tsl2583_als_calibrate() to return 0 on success

tsl2583_als_calibrate() returns the newly computed gain_trim if the
calibration was successful. This function is only called by
in_illuminance_calibrate_store() and the return value inside that
sysfs attribute is only checked to see if an error was returned.
This patch changes tsl2583_als_calibrate() to return 0 on success.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>