Avoid detecting disconnection in MM when the data port is a TTY

When the modem uses a TTY for data connection, we must rely on the upper layers triggering the bearer object disconnection, so that the access to the TTY port is synchronized between e.g. pppd and MM.

If we enable +CGACT? polling or +CGEV events we could detect disconnections ourselves, but we would try to reuse the TTY without knowing whether upper layers are still using it.

Avoid this mismatch, because getting out of sync ends up messing the TTY settings (e.g. unexpected CLOCAL configuration) and we may end up getting HUPs due to the modem control lines, which would otherwise be ignored by MM.