staging: comedi: ni_65xx: tidy up the port<->chan conversions

Introduce some convienence macros to handle the port to channel,
channel to port, and channel mask calculations based on the 8
channels per port of the hardware.

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>