broadband-modem: track primary/secondary ports while enabling/disabling 3GPP URCs

We should not blindly assume that peek_port_primary() always returns a valid port, because the device may have been ejected while the AT sequence is running.

Instead, keep a valid port object reference in the operation context, and use it in all the AT command requests. The requests will fail if the device has been ejected, but the port object reference will be valid and the daemon won't crash.