staging: comedi: ni_mio_common: refactor gpct to stc register handling

staging: comedi: ni_mio_common: refactor gpct to stc register handling

The NI General Purpose Counter Timer (gpct) registers are mapped to the
STC registers with a big switch statement. Create a lookup table to handle
the mapping ot the registers. This is easier to maintain.

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>