iio: tsl2x7x/tsl2772: avoid potential division by zero

It may be possible for tsl2772_get_lux to return a zero lux value
and hence a division by zero can occur when lux_val is zero. Check
for this case and return -ERANGE to avoid the division by zero.

Detected by CoverityScan, CID#1469484 ("Division or modulo by zero")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>