usbtmc: more sanity checking for packet size

A malicious device can make the driver divide ny zero
with a nonsense maximum packet size.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20190820092826.17694-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>