staging: comedi: ni_tio.h: remove 'extern' from exported function prototypes

The 'extern' is not required, remove it.

Tidy up the function prototypes a bit.

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