iio: chemical: sps30: fix a loop timeout test

The "while (tries--) {" loop is a postop so it exits with "tries" set
to -1.

Fixes: 232e0f6d ("iio: chemical: add support for Sensirion SPS30 sensor")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>