USB: metro-usb: fix tty_flip_buffer_push use

commit b7d28e32c93801d60c1a7a817f774a02b7bdde43 upstream.

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

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