USB: serial: xr: avoid requesting zero DTE rate

When the requested line speed is B0 (hangup) there is no need to use the
current speed in the line-coding request. This specifically avoids
requesting a zero DTE rate when the current speed is B0, which could
potentially confuse buggy firmware.

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