staging: comedi: usbdux: allocating too much data

We only need to allocate enough space for a pointer.  We allocate the
space for the urbs themselves with the call to usb_alloc_urb() a few
lines later.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>