staging: comedi: ni_65xx: remove board() helper function

This local helper function is a duplicate of the comedi core
privided comedi_board() helper. Use that function instead and
use a local variable to hold the boardinfo pointer instead of
calling the helper each time the boardinfo is accessed.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>