staging: comedi: pcmuio: pointers should be cleared with NULL

Pointer variables should be cleared with NULL on 0. This quiets
some 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>