staging: comedi: pcmmio: quiet NULL pointer sparse noise

The comedi_async 'inttrig' member is a pointer to a callback
function. NULL should be used to clear it not 0.

This quiets a number of sparse warnings about:

Warning: Using plain integer as NULL pointer

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