They got removed:
commit 003948c54e5b5034a9bbb4923336f5aba125eae6
Author: Benjamin Gilbert <benjamin.gilbert@coreos.com>
Date: Tue Jan 23 18:06:30 2018 -0800
USB: serial: keyspan: Drop firmware Kconfig options
The USB_SERIAL_KEYSPAN_* firmware options no longer do anything.