usb: serial: Remove unused tty->hw_stopped

The tty core does not test tty->hw_stopped; remove from drivers
which don't test it themselves.

Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>