device manager: Move filter for supported devices to the scan routine

Move the check for supported monotonic rate devices from the call site
into the actual scan routine.

The scan routine already used to (store and) return a variable length
set of found devices including none, and call sites can cope with this
situation. Pending extensions may call the scan routine several times,
and callers shall not duplicate the extra test condition. While we
expect the specific test for what's supported to change in the future.