staging: comedi: ni_660x: refactor ni_gpct_to_660x_register()

Convert this big switch into an array and refactor ni_660x_gpct_{write,read}()
functions to use the array to find the register offset.

All the TIO (GPCT) registers are included in the array except for NITIO_G0_ABZ
and NITIO_G1_ABZ. These registers only exist on the ni_pcimio m-series boards
and this driver will never read/write them.

Just in case someone adds a new entry to the enum ni_gpct_register in ni_tio.h,
add a dev_warn() for any unhandled registers.

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>