usb: Don't init pointer to zero, but NULL

The pointer should always be inited to NULL, not zero (0). These are
two different things and not necessarily equal.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Stephen Warren <swarren@nvidia.com>