USB: cdc-acm: fix TIOCGSERIAL flags

The driver reports that it always uses a low-latency mode by returning
the ASYNC_LOW_LATENCY flag through TIOCGSERIAL.

Even if this behaviour could not be changed, this may have made some
sense prior to 7a9a65ce ("cdc-acm: Fix long standing abuse of
tty->low_latency") which removed the unconditional setting of the
corresponding tty low_latency flag (something which had always been
broken in itself).

Since the driver does not have a low-latency mode, let's drop the flag.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>