staging: iio: tsl2583: updated code comment to match what the code does

If channel 0 does not have any data, then the code sets the lux to zero.
The corresponding comment says that the last value is returned. This
updates the comment to correctly reflect what the code does. It also
clarifies the comment about why 0 is returned.

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