staging: comedi: ni_6527: tidy up ni6527_di_insn_config()

The core will validate the insn->n value based on the actual instruction
(data[0]) that is being handled. Remove the sanity check and change the
instruction handling into a switch. This follows the normal format for
(*insn_config) functions and make adding additional instructions easier.

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>