The callback is now hooked up for any USB to serial driver that wants
it.  We only register the callback if any of the usb-serial structures
want it, this keeps the USB core happy.

Thanks to Alan Stern for the ideas on how to do this.

Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>