iio: isl29018: fix to return error or 0 in isl29018_write_raw()

We had assigned the return value to 'ret' but ignored it when
return from isl29018_write_raw(), it's better to return 'ret'
instead of 0.

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>