staging/ozwpan: Fix NULL vs zero in ozusbsvc.c (sparse warning)

This patch fixes the warning "Using plain integer as NULL pointer",
generated by sparse, by replacing the offending 0s with NULL.

If the initialization with NULL was unnecessary (due to unconditional
assignment before first use) it was removed.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>