staging: comedi: drivers: usbdux.c: fix DMA buffers on stack

fix for instances of DMA buffer on stack(being passed to usb_control_msg) for
the USB-DUX-D Board driver.

Signed-off-by: Kumar Amit Mehta <gmate.amit@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>