staging:iio:light:tsl2563 channel spec buglet / always reading same adc.

The IIO_LIGHT channel was not marked as being a processed_val despite
clearly being in lux.

The IIO_INTENSITY channel reads were dependent on channel and that isn't
specified for either adc (as they now use modifiers).  Hence use the
modifier instead.

Reported-by: Jon Brenner <jbrenner@taosinc.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>