USB: serial: fix up reset_resume callback

If the usb-serial driver doesn't have a reset_resume callback, then we
need to tell the USB core that it doesn't, and it needs to rebind the
device.

Thanks to Alan for pointing out my mistake, and providing the fix.

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