USB: ftdi_sio: fix problem when the manufacture is a NULL string

On some misconfigured ftdi_sio devices, if the manufacturer string is
NULL, the kernel will oops when the device is plugged in.  This patch
fixes the problem.

Reported-by: Wojciech M Zabolotny <W.Zabolotny@elka.pw.edu.pl>
Tested-by: Wojciech M Zabolotny <W.Zabolotny@elka.pw.edu.pl>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>