USB: ftdi_sio: remove tty->low_latency

Fixes tty_flip_buffer_push being called from hard interrupt context with
low_latency set.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>