api: expose ignored ports as MM_MODEM_PORT_TYPE_IGNORED

At the moment, ignored ports show up as (unknown) in the ports list in mmcli. This makes it look like something went wrong while probing.

Actually ModemManager already tracks unknown and ignored ports separately (MM_PORT_TYPE_UNKNOWN vs MM_PORT_TYPE_IGNORED) but the API always exposes them as MM_MODEM_PORT_TYPE_UNKNOWN. Add MM_MODEM_PORT_TYPE_IGNORED and use this for ignored ports so they show up as (ignored) instead in mmcli.

e.g. with qcom-soc, before:

           |             ports: rmnet0 (net), rmnet1 (net), rmnet2 (net), rmnet3 (net), 
           |                    rmnet4 (net), rmnet5 (net), rmnet6 (net), rmnet7 (net), 
           |                    rmnet_usb0 (unknown), rpmsg0 (qmi), rpmsg1 (at)

after:

           |             ports: rmnet0 (net), rmnet1 (net), rmnet2 (net), rmnet3 (net), 
           |                    rmnet4 (net), rmnet5 (net), rmnet6 (net), rmnet7 (net), 
           |                    rmnet_usb0 (ignored), rpmsg0 (qmi), rpmsg1 (at)

Originally I made the MR on top of the qcom-soc branch but it seems to apply on top of master without any conflicts. It's a rather minimal change :)