staging: comedi: pcl818: define the mode register bits

Define the bits in the register to remove the magic values.

Tidy up pcl818_ai_cancel(). The 0x73 mask of the value read from
the control register will disable dma and interrupts but it does
not change the trigger mode. So the software trigger following it
might not work.

Just disable the trigger and clear any pending end-of-conversion.

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>