USB: serial: keyspan: Drop firmware Kconfig options

The USB_SERIAL_KEYSPAN_* firmware options no longer do anything.

Fixes: 5620a0d1 ("firmware: delete in-kernel firmware")
Signed-off-by: Benjamin Gilbert <benjamin.gilbert@coreos.com>
Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>