USB: serial: ftdi_sio: Handle the old_termios == 0 case e.g. uart_resume_port()

  Handle null old_termios in ftdi_set_termios() calls from uart_resume_port().

Signed-off-by: Andrew Worsley <amworsley@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>