staging: comedi: s626: pointers should be cleared with NULL not 0

pdma->LogicalBase is a void *, NULL should be used to clear it not 0.

This quiets a sparse warning 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>