USB: legousbtower: zero driver data at allocation

Zero the driver data at allocation rather than depend on explicit
zeroing, which easy to miss.

Also drop an unnecessary driver-data pointer initialisation.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191105084152.16322-4-johan@kernel.org


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