iio:chemical:sps30 Supress some switch fallthrough warnings.

Fixes warnings reported on linux-next but marking one path
and adding an explicit return in the other.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Andreas Brauchli <a.brauchli@elementarea.net>
Acked-by: Tomasz Duszynski <tduszyns@gmail.com>