staging: iio: light: Fix quoted string split across lines

This fixes the following checkpatch.pl warning:
WARNING: quoted string split across lines

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>