staging: comedi: amplc_pci230: Replace NULLFUNC

The source code defines a macro NULLFUNC

  #define NULLFUNC 0

and uses it as a generic null function pointer constant.  This is
superfluous.  Just use NULL instead.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>