staging: wfx: replace 0 by NULL

Replace 0 by NULL as the return value of a pointer-returning function.
Issue detected by sparse tool.

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Link: https://lore.kernel.org/r/20191105010352.222479-2-jbi.octave@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>