staging: comedi: amplc_dio200_common: remove 'which' from struct dio200_subdev_8254

This member is only used in the "set gate" and "set clock" helper functions. Remove
it and calculate the value when needed.

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>