USB: remove incorrect struct class_device from the printer gadget

This field does nothing, and should not be allowed to stick around
incase someone gets any other ideas...

Signed-off-by: Tony Jones <tonyj@suse.de>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>