staging: comedi: s626: define S626_LP_CNTR* registers based on channel number

Redefining the Counter Preload/Latch registers as a macro that calculates
the register offset based on the comedi channel number.

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>