USB: metro-usb: fix tty_flip_buffer_push use

Do not set low_latency flag at open as tty_flip_buffer_push must not be
called in IRQ context with low_latency set.

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