broadband-modem-cinterion: keep operator and mode

Currently when requesting a specific operator with --3gpp-register-in-operator=26201 and additionally set the allowed modes with --set-allowed-modes=4G. The previously set operator 26201 is overwritten due to the architecture of the AT+COPS command (AT+COPS=,,,). The same thing happens if the allowed mode is set first and then an operator is requested.

Also the cinterion plugins does not implement load_current_mode which makes the device have a different 'current mode' state than MM after initialization and prevents setting the device's allowed mode in some cases -> Fixes #926