Elements in a DBus interface name cannot start with a digit, so
"org.freedesktop.ModemManager1.Modem.3gpp" is an invalid interface name.
Renamed here all relevant interfaces so that they have an additional "Modem"
prefix in the element, so we get now:
"org.freedesktop.ModemManager1.Modem.Modem3gpp"
"org.freedesktop.ModemManager1.Modem.Modem3gpp.Ussd"
"org.freedesktop.ModemManager1.Modem.ModemCdma"
Objects generated with gdbus-codegen maintain the previous names.