staging: comedi: ni_660x: remove ni_gpct_to_660x_register[]

enum ni_gpct_register and enum ni_660x_register now have a 1:1
relationship for the NITIO_* registers. The static const array
is no longer necessary to find the proper NI660X_* register for
a given NITIO_*. Remove it and refactor the register read/write
functions.

Use the NITIO_* values to init the ni_660x_reg_data[] array and
remove the unnecessary NI660X_* enum values.

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>