iio: light: si1133 fix test for val being not zero or not one.

The current check on val always results in true and so the
call to sii1133_update_adcsens never gets called. Fix this check
so it returns with -EINVAL only when val is not zero and not one.

Detected by CoverityScan, CID#1472099 ("Logically dead code")

Fixes: e01e7eaf ("iio: light: introduce si1133")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>