iio:light:cm3323: replace unneeded variable

In cm3323_read_raw() i is used as return variable for the integration time
index. The also existing return variable ret however is unused in this
case, although appropriate. Replace i with ret and drop it.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>