staging: comedi: pcl818: define the status register bits

Define the bits in the status register and remove the magic values.

Writing any value to the status register clears any pending interrupt.
For aesthetics, rename the status register and remove the "clrint"
register.

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>