broadband-bearer: don't close the port when we couldn't open it

If data_flash failed because the port couldn't be opened (perhaps it went away), trying to close it would trip an assertion failure:

Modem /org/freedesktop/ModemManager1/Modem/0: state changed (connected -> disconnecting) (modemu) could not open serial device (2) _close_internal: assertion 'self->priv->open_count > 0' failed