staging: comedi: ni_daq_dio24: local symbols should not be exported

The symbol dio24_cs_driver is only referenced in this file. Make
it static.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>