staging: comedi: ni_6527: tidy up ni6527_intr_insn_config()

staging: comedi: ni_6527: tidy up ni6527_intr_insn_config()

Handle the instruction (data[0]) with a switch to make it easier
to add additional instructions in the future.

To clarify the code, factor out the code that sets the rising/falling
edge detection.

Rename the CamelCase defines used for the rising/falling edge detection
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>