staging: comedi: pcl818: tidy up the remaining register defines

For aesthetics, rename the timer/counter enable request register
and define its bits.

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>