lib/wp/device: demote missing SPA warning to message

Warnings can be scary, so best not to scare users with what's likely
intentional omission of a particular SPA plugin (currently V4L & BlueZ).
6 jobs for SpaNilCheck in 1 minute and 37 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #13683785

00:00:26

passed container_ubuntu #13683786

00:00:26

 
  Build
passed build_on_fedora_no_docs #13683788

00:01:03

passed build_on_fedora_with_docs #13683787

00:01:09

passed build_on_ubuntu_no_gir #13683790

00:01:04

passed build_on_ubuntu_with_gir #13683789

00:00:34