base-manager: quiet message about unsupported ports

This message gets printed for all netdevs and ttys, including most
machines normal ethernet/Wi-Fi interfaces.  It seems a bit less critical
than 'warning' level would indicate.

ModemManager[32097]: <warn>  Couldn't find support for device at '/sys/devices/pci0000:00/0000:00:19.0': not supported by any plugin
ModemManager[32097]: <warn>  Couldn't find support for device at '/sys/devices/pci0000:00/0000:00:1c.1/0000:02:00.0': not supported by any plugin
1 file changed