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

commit c515598e0f5769916c31c00392cc2bfe6af74e55 upstream.

  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>
Cc: Lotfi Manseur <lotfi.manseur@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>