can: peak_usb: fix slab info leak

Fix a small slab info leak due to a failure to clear the command buffer
at allocation.

The first 16 bytes of the command buffer are always sent to the device
in pcan_usb_send_cmd() even though only the first two may have been
initialised in case no argument payload is provided (e.g. when waiting
for a response).

Fixes: bb478555 ("can: usb: PEAK-System Technik USB adapters driver core")
Cc: stable <stable@vger.kernel.org>     # 3.4
Reported-by:  <syzbot+863724e7128e14b26732@syzkaller.appspotmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>