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

This member of the private data is set but never used. Just remove it.

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>