data_iface is the serial port over which PPP should be run, so
we need to preserve that and not overwrite it with the PPP interface
name.  When reconnecting, pppd wants the TTY to run PPP over (eg the
ModemManager data_port like ttyUSB0) but if we overwrote that with
ppp0 on the last connection, that's extremely unhelpful and pppd will
fail to start.