staging: iio: tsl2583: add error code to sysfs store functions

in_illuminance_input_target_store() and in_illuminance_calibrate_store()
validated the data from userspace, however it would not return an
error code to userspace if an invalid value was passed in. This patch
changes these functions so that they return -EINVAL if invalid data is
passed in.

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