staging: comedi: adl_pci9118: remove 'dmabuf_panic_size' from private data

This member of the private data is only used in some #if 0'ed code. Remove
it along with the unused code.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>