staging: comedi: ni_pcidio: local functions should not be exported

The function ni_pcidio_event() 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>