staging: comedi: pcl818: use subdevice (*cancel)

Use the subdevice (*cancel) operation to remove the need for a forward
declaration.

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>

diff --cc drivers/staging/comedi/drivers/pcl818.c
Simple merge