staging: comedi: me_daq: tidy up counter registers

For aesthetics, convert the counter registers into macros that take
the counter channel and return the correct register offset.

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>