iio: light: signedness bug in stk3310_write_raw()

"index" needs to be signed for the error handling to work.

Fixes: be9e6229 ('iio: light: Add support for Sensortek STK3310')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>